It's been a big day for Ross. He, along with some 20m other Americans, has just found out that he's the father of Rachel's baby. I feel I should buy David Schwimmer a drink to celebrate. Instead, he offers to buy me a hot dog. We are in Chicago, where Schwimmer - whiny, neurotic Ross Geller in Friends - studied drama and founded a theatre company. It's his week off from Friends and he's helping raise funds for the Looking Glass Theatre's new home. They have been peripatetic for the last 12 years, and they are about to move into a big stone castle that serves as the city's water pumping station.

Last night, Schwimmer hosted a 'cultivation' evening to attract potential investors and schmooze the city's business interests. The opportunity to meet 'Ross from Friends' makes it a hot ticket, but Schwimmer is more than a front man for the theatre. It's very much a labour of love and, in addition to giving them financial support, he writes, directs and stars in their productions. Unlike his five sitcom co-stars, Schwimmer has a theatrical background - and a keen interest in social issues. Right now, he's writing scripts on the subjects of race and internet paedophilia. In real life, he is a very active director of the Rape Foundation in Los Angeles, lecturing about date-rape drugs at colleges in the city.

It's swiftly clear that Schwimmer is no pampered Hollywood star, despite the $750,000-an-episode pay cheque from Friends and the obligatory multi-million-pound mansion in Los Angeles to go with his Chicago loft. He makes his own schedule for our interview, leaving his home number on my hotel voicemail even before we've spoken; he wears clothes so nondescript that I can't even remember what they were, he drives me around in his eight-year-old car and he invites me to play volleyball with him and his friends. We don't go to stylish, exclusive places; he suggests a hot-dog place before settling on a neighbourhood restaurant called Toast, moving on later to cheeseburgers at his local bar. He has no minders or assistants, his publicist is a childhood friend (who is miles away in New York) and he turns off his cellphone while we're talking. I'd like to tell you something bad about him just for the sake of balance: he says he sometimes steals parking places while other drivers are about to reverse into them, and he carries a big, heavy torch behind his car seat in case they get violent. But he has yet to use it in anger. And he wouldn't sign an autograph on the way to lunch. But only because he didn't have a pen. And he was very sorry.

'There's nothing negative to tell about David,' says British actor Andy Nyman, who has just spent several weeks working with Schwimmer on Uprising, a film about Jewish resistance fighters in the Warsaw Ghetto, shot in Bratislava. I'd called Nyman in search of some balance, but failed to find any. 'I can see that when you are as rich and famous [as Schwimmer] it's very easy to be difficult. But he's a proper actor: diligent, hard-working, and truly a lovely man. He's great at dealing with his celebrity, too.'

After eight years in what quickly became the most popular sitcom in America, David Schwimmer is one of the most recognisable faces in show business. But, while he may be adept at dealing with his fame, he is still uncomfortable with it. 'One of the ways I stay grounded, I like to think, is by having little mini-battles every day,' he says. When the staff at Starbucks recognise him, for example, and tell him to jump the queue, he insists on waiting his turn. 'Sometimes,' he tells me, 'if I'm in a bar, guys will want to buy me shots. I'm not a big drinker but if I say no, it's like I've insulted them. They're like, "Ooooh, think you're too good to drink with me, do ya?" And then I'm an asshole.'

But it's worst when he's with someone else. 'Whoever I'm with just becomes invisible,' he explains. 'Literally invisible. People push them out of the way to get to me. Sometimes they grab my arm and start yelling, "Ross, Ross!"' He's really very agitated now; he grabs my arm to demonstrate, and knocks it right off the table. 'I'm sorry,' he says, mortified by his display of anger. 'Did I hurt you?'

At almost 6ft 2in, and with an athletic physique honed by daily workouts, yoga and weekly volleyball games, Schwimmer can deal with day-to-day hassle without too much difficulty. But he despairs of celebrity culture, of people becoming famous for being on Jerry Springer and taking part in 'reality' TV shows. 'I find it sad that everyone wants their 15 minutes of fame and, in America, we are so consumed by it. It has never been comfortable to me. I don't hang with a lot of celebrities. And I don't go to Hollywood parties. I hear about these parties at big directors' and stars' and producers' homes and I guess I'm not really interested.' He admits: 'To be honest, in some respects it has probably hurt my career because I don't kiss ass. I don't show up and get my face out there all the time.'

Despite being brought up in Los Angeles, Schwimmer has always been an outsider there, just as his parents were when they emigrated from New York with their two young children. At school, he was the outsider, too. He confesses to having been a troublemaker and a bully in his younger days. Nor did he fit in when he was sent to the ultra-glamorous Beverly Hills High School. 'When I was there I always felt: this is not me, I'm surrounded by people with a different value system. And I just wanted to get out of California.' He left to study drama at Chicago's Northwestern University and his friends today are the people he's known from school and college, his theatre friends; his publicist, Ina, went to infant school with him.

Until recently, Schwimmer used to live in Hollywood, but he couldn't stand it. 'It was too noisy, too busy,' he says, elaborating with a really Ross-like inflection. 'Too many cool people trying to be really cool.' Now he lives, alone, in a big house in Hancock Park. 'It's a mixed ethnic community, which is rare in LA, and although there are a few celebrities in the area, it doesn't feel like an industry town,' he says. 'It's like a regular neighbourhood where you see kids playing in the street.' He lives there because it reminds him a lot of Chicago, where he has had a home for the past 15 years, and he often has his Chicago friends to stay at his big, barely furnished '1924 Spanish' home, which he is slowly restoring.

David Schwimmer was born, a year after his sister Ellie, in the New York borough of Queens in November 1966. Two years later, his parents, Arthur and Arlene - both lawyers - moved to Los Angeles, where his mother became a leading light in the fledgling feminist movement. 'I have unusual parents,' he explains. 'They were always involved in issues. My dad has just retired and the first thing he did is join this organisation called Habitat for Humanity, which Jimmy Carter started years ago. Basically, my dad was a lawyer sitting behind a desk for many, many years, and the first thing he did on retiring was throw on a tool belt and go into the outside of the communities and build for those in need. You gotta see him - he looks like he should be buried in books, but he was so excited to be actually physically giving back when before he could only give back either monetarily or intellectually. That's the kind of family I come from.'

They were, he says, a very warm family. 'I always remember just a lot of love, playing catch with my dad, going to plays and movies. It was a fun family, always card games after dinner and stuff like that. But at the same time it was really rigorous and there was a lot of pressure on academic achievement.'

Schwimmer was tall and lanky and hyperactive. 'I was always the class clown, I was the troublemaker in class. And I was always restless.' He got into a lot of fights. 'I was a bully,' he remembers. 'I was nearly expelled from junior high and elementary school before that.' One time, he faced expulsion after a brutal fight with a tomboy who was bigger than him, and his mother came to see the principal to ask why the girl was not getting the same punishment as her son. David was impressed because Arlene was a prominent force in the original women's liberation movement, founding the National Organisation for Women in California and the League of Women Voters. That day, she demonstrated something vitally important to him. 'She believed in equal rights.'

As his mother's career as a divorce lawyer became increasingly successful, thanks to high-profile clients including Elizabeth Taylor and Roseanne Barr, the Schwimmers moved to Beverly Hills and sent David to the famous high school, popularised in Beverly Hills 90210. Puberty played havoc with his looks; he became short and fat, he had a premature moustache and braces fastened by a device that he likens to 'a harness for a horse'. Feeling no kinship with most of his overprivileged classmates, he buried himself in studies. He was best at science and maths and thought he would become a doctor. He became popular within his clique - 'the dorky science and math guys' - because he also did drama. 'And within the acting group I was also kinda popular cos I was funny and also doing well in school. Somehow within those two cliques I found friends.'

But he was still an outsider. 'Within the cool cliques I was a complete outcast. There was this huge group of very beautiful people at high school: really gorgeous men and women who had the latest fashions, drove the new BMW or Porsche or Mercedes to school - the Volkswagen Rabbit convertible was really popular one year. Those were the kids who had parties after school in big mansions in Beverly Hills. My friends and I were never part of that.'

When Schwimmer finally got his driving licence in his last year at school, his parents bought him a car. 'They knew I was leaving for Chicago very soon, so they weren't going to buy me a new car. So my dad went shopping at used car lots and we found a 1976 Chevy Monte Carlo. It was literally shit brown.' One Saturday night, Schwimmer and his friends went out in this 'boat of a car' and drove passed a party that the cool people were throwing at one of their houses. 'Just driving by this incredible house, we heard people laughing and we recognised some of the beautiful people from school and we weren't invited and certainly weren't going to crash it - you know, peer pressure and everything - we just didn't want to be subjected to that kind of ridicule.'

Schwimmer realised he was always going to be the outsider. 'I definitely felt a class system in place.' He felt the same thing when he later started trying to find work and discovered his looks were considered 'too ethnic' for leading roles. He thinks Hollywood is rife with racism and anti-semitism, even though many of the producers and directors are Jewish. The only time he can recall Ross being defined by his race/religion in eight years of Friends was when he dressed up for his son as the 'Hanukkah armadillo'. Schwimmer was outraged when he watched the movie Star Wars: Episode 1 to see 'a racist stereotype: a small schmizerly pawnbroker with a huge nose - all the characteristics and traits of a Jew. I remember wondering whether anyone else had seen this and was feeling what I felt. I think it's damaging.'

Schwimmer believes that racism already 'saturates' America and worries that it is becoming legitimised since 11 September, a theme he wants to tackle in his stage adaptation of Race, a massive collection of testimonies on the subject compiled by the broadcaster, author and journalist Studs Terkel. 'I don't think there's a real concentrated effort to alleviate the problem of race in America,' Schwimmer says. Of equal concern to him are internet predators, about whom he is accumulating case histories before writing his script. 'I have stacks of cases and more than half of them are teachers, and two-thirds are married men with children,' he says with evident disgust. He has even logged on to chat sites, posing as a teenage girl, and was horrified to find 'she' was propositioned within minutes. His drama will also tackle the wider issue of how the internet has changed the family. 'The family dinner discussion around the table has disappeared. Kids run to their rooms now and spend hours either playing games on the computer or chatting with people.'

Clearly his dramatic interests are a far cry from the frivolity of Friends. And, although the sitcom has earned him Ross-like leading roles in comedy films such as The Pallbearer (with Gwyneth Paltrow) and Kissing a Fool, Schwimmer recently took a decision 'to make an effort to find roles that are as far away from the character of Ross as possible'. Television viewers have just seen him as a sadistic drill instructor who turns out to be an inept leader of men in Spielberg's Second World War epic Band of Brothers.

Schwimmer had five great-uncles who all fought for the US in the war and, unlike Private Ryan's siblings, all survived. 'For a lot of veterans, this was the highlight of their lives; they felt there was a value to their lives for the first time and it had an importance greater than themselves,' he says, admiringly. He believes it would be unwise to talk directly about the war on terrorism, but in a hypothetical draft he would answer his country's call 'without hesitation'. Things may be 'forlorn and fucked-up' in terms of education and racism and health care, he says, but you've still got to defend your freedoms. 'You have to inherently believe what the country stands for, or else you shouldn't live here.'

In the forthcoming Uprising, he plays a Jewish police officer working for the Polish police, who faces the stark choice of betraying his race or working for the freedom fighters. Next, he will be seen as 'a young, immoral, independent film producer' in Mike Figgis's experimental film Hotel, shot with a huge cast in Venice. It was, he says, 'the greatest experience ever'. He's seen it and it looks beautiful: full of 'really unforgettable images and moments'.

When he was 12 or 13, Schwimmer went to a Shakespeare workshop given by Ian McKellen in Los Angeles. 'I was absolutely riveted,' he recalls. He started studying drama at Beverly Hills High. There was a local drama contest called the Southern California Shakespeare Festival. Schwimmer entered three years running. 'I won first place in a group scene and also first place in an individual dialogue, which I adapted from The Tempest. It was a scene with myself, basically; I played three characters simultaneously - Prospero, Ariel and Caliban.' He wanted to go straight into acting, but his parents insisted he go to college so he had something to fall back on in case the acting didn't work out. His exam results were so good that he had his pick of a dozen universities. He chose Northwestern because he had attended its summer drama course when he was 16 and felt it had the best undergraduate programme in the country.

After graduating in theatre and speech, his first TV role was in LA Law in 1986, followed by the comedy-drama series The Wonder Years. He had a recurring role as a lawyer-turned-vigilante in NYPD Blue and appeared briefly in ER in 1993, before auditioning, unsuccessfully, for a series pilot called Couples. He tried again when it was rewritten for a cast of singles, and retitled Friends. The rest is history. Soon, he hints, Friends really will be history. 'Our contract is up in April, we're not signed up to do another year and we are not even in talks,' he reveals. 'No one wants to call it quits, but I feel like there is so much other stuff I want to do - theatre and film and developing other television as well - that I'm ready for it to be over. I wouldn't mind if the show was over after this year.' It's clear that he's had enough.

People are always wondering whether Schwimmer is like Ross. Someone once told him that Ross takes things more seriously than the other characters and that he feels things more deeply than Joey and Chandler. 'He's literally in a different dimension, a different reality than the others. Not everything is frivolous to him, there's a seriousness about him in terms of how he thinks about relationships, the family; he's more issue oriented - if there were issues on the show!' He laughs. 'And that's more me.'

Schwimmer says that he, unlike Ross, has changed over. But he manages to remain 'intellectually engaged' by directing some of the episodes. 'So it's not mindless. There's a challenge in itself in trying to keep things fresh, playing the same character for eight years.' He adds, somewhat unconvincingly: 'It's actually quite fun.' Unlike the stint where Ross was engaged to an English girl. His lip curls instinctively at the mention of Helen Baxendale, and for once the Friends code of silence is broken. 'To be honest with you I don't think she wanted any part of us,' he says. 'I felt like I, at least - and the others - welcomed her with open arms into the family, but she kept a real professional and personal distance. I was disappointed that she didn't want to engage. Some kind of connection would have been nice - after all, we were going to be married.'

He insists, as they all do, that the six stars of Friends really are good friends, and that, because they became famous together, they are able to keep each other's egos in check, though they socialise less off-set than they once did now that most of them are in long-term relationships.

Schwimmer's own, with Israeli actress Mili Avital, has recently come to an end. He blames the tabloids for printing false stories that they had become engaged at a particularly difficult point in their relationship. 'It was really hurtful,' he says bitterly. 'They just don't understand the damage they cause.'

He has not had a date in the three months since they broke up, and there's a sadness in his eyes when he talks about the split. 'She's incredible, simply the best person I've ever known, inside and out. She's the most genuine, and generous, and artistic and clear-headed person I've ever known.'

It has been suggested that Schwimmer, who had a three-year relationship with law clerk Sarah Trimble in the mid-90s and has dated singer Natalie Imbruglia, has a problem with commitment. But he disputes that. 'I was committed to Mili for four years. I think, if anything, I'm a workaholic and that was the problem. I was not able to find a healthy balance between my work life and my personal life. It's a flaw in my character and I can't seem to find the balance yet.'

He is coping by burying himself in more work and believes he will 'flip a switch' when the time comes to start a family. 'I know myself very well and I love kids and I know that, when I have them, I'm going to want to be there, not making films. I will want to have a break. And I want to have more than one child. I look forward to it, but I'm just not ready to change my lifestyle that drastically.'

That time, however, may be just around the corner. 'Maybe it's as simple as waiting for the show [Friends] to end and maybe moving back here to Chicago and spending more time with the theatre company and deciding to raise a family and direct and act in the theatre. Maybe it's that simple, that that time will come, maybe in another year or two, and my life will change,' he says, optimistically.

The change could be a drastic one. 'I've also thought about just totally leaving this profession,' he says, seriously. 'I've made a good amount of money. I'm very happy that I can now support my theatre company and support friends and family, and I'm ready to maybe go back to school and change careers.' It can't be the drink talking: Schwimmer has nursed a single pint of beer for almost two hours now. 'I've thought about it a lot,' he says, talking enthusiastically about going back to college to train as a teacher.

'I think we can really use more - and better - teachers in this country. I would like to teach in public [state] school. I think I have a lot to offer in that area.'

He gets excited running through the subjects he might teach - English, maths, science, history ('I'm a history buff!') and he seems completely serious. 'I just see this chapter of my life coming to a close and I'm not quite sure what the next chapter has in store. But until I figure that out, and decide what I'm gonna do, I'm certainly not ready for the family chapter to begin yet. One of my dreams has come true: I'm making a living at what I love to do most. I'm really blessed. Not only that but I can retire - and I'm not even 35. So now isn't it time to maybe do something else and give something back?'

· The final episode of Band of Brothers will be shown this Friday on BBC2 at 9pm. The new series of Friends will be shown on E4 from January 2002.