Piers Morgan used to say of the actor Nicholas Lyndhurst that he may have been incredibly famous – this was back when Lyndhurst was an on-screen ubiquity as Rodney Trotter, as a time-traveller in Goodnight Sweetheart and an occasional cross-dresser in adverts for Smiths – but how much did anybody know about his private life?

There were no nightclub ejections or illicit affairs, to anyone's knowledge, really no Lyndhurst-based muck at all, and Morgan's point was that privacy could be pretty well maintained by the famous if the famous really wanted it. It is tempting to wonder if we have a present-day equivalent – whether such a thing is even possible in an age of the blogging hospital receptionist, the dusk-to-dawn ITV2 reality show, the biannually updated autobiography.

One who has tried is Simon Pegg: actor, comedian, Hollywood weevil who seemingly has a role in every blockbuster series going, and deeply private man. Which is his paparazzi-ringed nightclub of choice? What colour is his wallpaper? Pegg, who first became prominent as the star of TV show Spaced, gathered acclaim in the adored zombie spoof Shaun of the Dead, is now an established figure in major franchises such as Star Trek and (soon) Steven Spielberg's Tintin. But who has he shagged?

"I have problems with my private life," Pegg says, over soft drinks in north London. Juicy, I think – but he doesn't mean clarinet-sized crackpipes or an addiction to lipo, rather problems when his private life is nudged public. "Work is work, and you have to do interviews to promote your stuff. I'm just conflicted about talking about private stuff, and when that comes up in interviews I tend to shut down."

Sitting opposite him I peer anxiously at my list of questions. There's one about how he first met his wife; another about being best buds with Chris Martin of Coldplay. Pegg looks pretty relaxed, a first of two Diet Cokes nearly drained, limbs spread on a leather sofa and his Star Wars-themed trainers propped up on a table. But I still try to angle away my notepad, and mentally panic-fashion a few extra questions about zombie films just in case.

"We didn't tell anyone my wife was pregnant or that she'd given birth," he says, "but journalists found out and it was printed – 'Shaun of the Dad', very nice headline, thank you the Mail. It's not like I'm pathologically private, and I don't really have any secrets, but I just feel you have to hold on to something of yourself."

The 40-year-old has made good use of a particular trick, in the face of too-intimate interviewers, in which he diverts personal questions into chit-chat about his dog Minnie. There was a recent interview in which Pegg was asked, "What's it like being a father?", to which he replied, "My dog likes eating socks." Minnie, he reasons, doesn't read Heat or watch the TV channel E! and her bosom secrets can be sacrificed.

It brings us both to a difficulty. I'm not desperately keen to talk for an hour about the digestive whims of a Miniature Schnauzer. And Pegg, rather surprisingly, has just written an autobiography. It is why he has agreed to be interviewed – the imminent publication of a 300-page memoir entitled Nerd Do Well, charting his rise from a precocious kid scooting around Gloucestershire on a Raleigh Grifter, to jobbing stand-up comedian in London, to Tim in Spaced and Shaun in Shaun and Montgomery "Scotty" Scott in Star Trek.

Britain has weathered a bit of a silly season for autobiographies: Peter Mandelson's near real-time account of the election last July, Tony Blair's long-awaited non-apology in early September, The Stig from Top Gear stirring absolutely nobody a fortnight later by revealing that under his identity-shrouding helmet he was an anonymous stunt driver of whom nobody had heard. Still, Pegg's book was a surprising sight in the catalogues, and having read an advance copy in the days before meeting him, I am suddenly in possession of all kinds of fascinating details I didn't expect to know.

There's the nervy thrill he got as a little boy when undressing his Lone Ranger action toy. The romantic break-up, in the mid-90s, to which he credits most of his career's oomph. The several near-encounters with his wife, Maureen, that preceded them meeting properly in an airport shuttle bus. Not to mention the circumstances of his first snog, grope, blowjob and shag (hooray!), this last event ushered in, apparently, by the back-to-back screening of Withnail and I and Evil Dead 2.

Even Nick Lyndhurst, in the end, sold a wedding to OK! magazine. Is this book a reverse of many years of principled privacy? "I didn't want to do a tell-all, stitch anyone up," Pegg tells me. "These are nuggets that I'm happy to part with; the kind of pub chat you share with people when you first meet them. Some people might want to hear stories from the set of Spaced or Shaun or whatever, but it's kind of boring. We went to work, we filmed, we had a laugh: there weren't any great adventures. I found that most of the interesting stuff was in the past."

So, for now, to the past. He grew up in Gloucester as Simon Beckingham, dad a key- board player in a local band and mum a member of an amateur dramatics troupe. The performative gene was passed down, and Pegg grew up a keen actor and joke-teller, obsessed with Jim Davidson and the musical Carousel, and in general a bit of a show-off. He had "a face that made aesthetic sense, as opposed to the frustrating Picasso adulthood has seen fit to furnish me with".

He became Simon Pegg in the mid-70s, after parental divorce and remarriage. The relationship between his mother and his stepfather was facilitated (crucial this) by Pegg's fledgling interest in sci-fi telly. Obsessed with the show The Six Million Dollar Man, he had asked his mum for a pair of red-and-white trainers like those worn by the programme's bionic man. His eventual stepfather ran the local shoe shop, and a first date with Pegg's mother was arranged on delivery of the bionic trainers to their front door. "Bouts of welcome unity" in a subsequently rocky relationship between Simon and Richard Pegg came, chiefly, when the two nerded-out together over films such as Raiders of the Lost Ark. For Pegg, cultural hobby-horsing and the demands of real life have always been fast entwined.

Star Wars has been the most consistent supplement and tonic, Pegg an impressionable eight-year-old when George Lucas's epic arrived in his local cinema in 1977. "I was the exact demographic for that film," he says. "I was a fucking 1m-high target, and it got me right through the eyes. I can mark out my life from that point in terms of my relationship with that film."

No exaggeration. A game of Jedi re-enactment in Gloucester's woodlands helped a youthful Pegg and his peers get over the death of a friend in a car accident. A socio- historical interpretation of the film with Ewoks as Vietnamese militia was his thesis subject as a film student at Bristol University. On one occasion, in hospital after an operation, Pegg awoke from his anaesthetically induced coma an hour too early, confounding doctors until it was realised he must have overheard a fellow patient on the ward watching Star Wars on VHS, jealously aroused by the sound of R2-D2's clicks and beeps.

The only influence given anything like equivalent life-defining status in his book is Nick Frost, a bearish east Londoner who has played Pegg's devoted, syrupy-witted best friend in much of their screen work together. It is a set-up that mirrors, foggily, the pair's relationship in real life. They were introduced by an ex-girlfriend of Pegg's who worked with Frost in a Mexican chain restaurant; she thought the two would get along, which they did, even though Pegg found Frost to be a kind of comedic Caliban – instinctively very funny, but pop-culturally illiterate. Unable to fathom a life without Harrison Ford, Doctor Who and Woody Allen, Pegg set about educating him in the ways of manic film and TV consumption.

It birthed an enduring friendship, close to the one they enacted on Spaced (all video marathons and PlayStation sessions and tea) which first broadcast on Channel 4 in 1999, with Pegg in the lead role as Tim and Frost as his friend Mike. The show, written by Pegg and co-star Jessica Hynes and directed by a then-unknown Edgar Wright, was bolstered by a prime Friday night spot just after Friends and became a hit, joining the pantheon of cult-adored TV that Pegg had long been a fan of himself.

At this point there was an early sign of his instinctive twitch towards privacy. Starting to get recognised, Pegg took to holing up of an evening in a scuzzy pub on the Archway Road, where he and Frost became such firm regulars they might take their shoes off to drink. One night they considered the hypothetical (and, you suppose, drunken) question: what would they do if zombies invaded their corner of north London right now? Frost imagined leaping across rooftops to a local football stadium, there to use the playing surface for farmland and enjoy a panoramic view of any approaching hordes. Pegg decided he'd stick in the pub and fight off the living dead from there. He made a mental note of the idea.

The girlfriend who introduced Pegg to Frost is named in his book, pseudonymously, as "Eggy Helen" (a private joke, says Pegg, but adding that in Spaced she inspired the character of Sarah, the woman who dumps his character in the first episode). In real life that relationship ended with "a small amount of blood and broken glass" when Pegg was dumped and, in response, put his fist through a glass window. Having to get his smashed knuckles repaired in hospital is a moment returned to time and again in the memoirs, a nadir in his life, but also a turning point. "It was a wake-up call," he says. "I was in a relationship, it was very comfortable, and I got sluggish. It was only when I got cut free that things got busy. I can track it from that point exactly – everything went 'Whoosh!'"

Whoosh: after two successful series of Spaced, Pegg made Shaun of the Dead with Edgar Wright as co-writer and director, and Frost as co-star. The film was set mainly in a pub just like the Archway boozer where the idea had first been born, but parochialism harmed it none, and Shaun won praise from the likes of Quentin Tarantino, Peter Jackson and zombie-film doyen George A Romero on its US cinema release. Dazzling endorsement made Shaun an absolute smash on DVD, selling so well in America that distributors Universal were later able to compile the astonishing statistic that 40% of US males aged 17 to 39 considered themselves fans.

Whoosh: one minute Pegg was joking to a British magazine that, despite this, he was hardly in a position to fly out to LA and make Mission Impossible III ("an imaginary blockbuster that I plucked out of the air to demonstrate my disdain for Hollywood ephemera"), the next he was cast as weapons technician Benji Dunn in Mission Impossible III. That film was directed by JJ Abrams, who, soon afterwards, went to work on a reimagining of the Star Trek film series, pinging Pegg a one-line email that said, "Do you want to play Scotty?" He did, and in the gaps between all this, starred in David Schwimmer's directorial debut Run Fatboy Run (2007) and How to Lose Friends and Alienate People with Kirsten Dunst (2008), as well as making another film with Wright, the action-movie pastiche Hot Fuzz (2007). Now his upcoming slate includes sequels to Star Trek and Mission: Impossible, a role in the next Narnia film and the John Landis-directed Burke and Hare, which is out in the UK next week.

Then there's the Spielberg film. One of the first people he told when he got the part, says Pegg, was his stepfather Richard, nodding back to their days of watching Indiana Jones films together. Due out in 2011, The Adventures of Tintin, directed by Spielberg and produced by Peter Jackson, will employ a kind of motion-captured animation similar to that used to realise Gollum on screen in Jackson's Lord of the Rings. Pegg will be rendered on screen as one half of Tintin's moustachioed detective partners, the Thompson twins. Frost, fittingly, will play the other twin. "It's the perfect way to represent our symbiotic careers," says Pegg. "We finally become the same person, appearing on screen with the same head."

Was it around the time of a stardom-confirming nod from Spielberg and Jackson that he began scribbling down incidents and anecdotes for the autobiography? No, says Pegg, he was reluctant at first and had to be talked into it by his publisher. The resultant work – an enjoyable read – was slowly lured from him over 12 weeks, Pegg parked at a computer next to his editor for seven hours a day. "I see these biographies come out, and there's a feeling of 'Who do you think you are?'" says Pegg. "It seems so awfully presumptuous to think anyone would give a shit about my life."

I am braced for the canine swerve at any moment. I have asked a question about his wife, and Pegg is reminiscing – with only minimal signs of sweaty agitation – about the first time they met.

It happened after several near-misses. Maureen, at the time a publicist for Sony Music, had been in the same place as Pegg on a few occasions without their actual meeting; prior to that she'd always felt mildly irked by him, from a distance, because of an ex-boyfriend's penchant for sacking off their Friday nights to stay in and watch Spaced. Booked on to the same holiday package to Thessalonica in Greece in 2000, Pegg and Maureen missed each other again in the departure lounge, then throughout a week in the city, finally how-do-you-doing on the shuttle bus riding home from the airport. They were married in 2005, and had a daughter last year.

"If I had matured in such a way that had stopped me thinking that sitting at the back of the bus was cool then I would never have met her," says Pegg. "But she went to sit at the back, too – she had the same impulses as me. That's quantum attraction in action."

Quantum whuh? It is a term that crops up in his book, and I ask Pegg to explain it a little more. The inner geek flares; his hands start making shapes. "It's a notion that attraction might actually be maths – but chaotic, fabulous maths. Not fate as some sort of ordained puppet master, rather a consequence of millions of micro-decisions happening simultaneously, drawing people towards each other. It explains how you come across the like-minded, even if it feels like coincidence, dependent on weird things you attribute to fate, but which might actually be assisted coincidence."

Yikes. "It's not as crystal-precise as a timepiece," he continues, "and it doesn't always work. Sometimes you're brought together, but you don't meet – Edgar and I worked out we were in the same cinema watching the same film five years before we met. Me and Maureen were at the same gigs or dating the same people, but always missing each other. You might get to within a metre of your soulmate, but for some reason that last little inch you don't make. I've met mine, and I'm very lucky. But the whole process of quantum attraction – it's like a drunk with a shotgun."

He really does speak like this, an infectious, sprawling swirl of ideas; the training, I guess, of many hundreds of hours of pub-talk with Frost. I find myself nodding and wincing and agreeing and wanting to order a couple of pints up to continue the discussion with the dictaphone off. But it stays on, and I'm emboldened enough by Pegg's candidness to ask about his friendship with Chris Martin. It seems significant, somehow, that two unassuming West Country boys should enjoy such outrageous success, having known each other before they both became globally famous. Is this quantum… "It's the quantum attraction thing again," says Pegg. "We met because we were both going to do what we did, because we were on a similar path."

They chummed up about a decade ago, meeting first at a Toploader gig Pegg attended through Maureen's work and bonding over a love of This is Spinal Tap, plus a shared awkwardness in the face of increasing public renown. Their career trajectories had followed a similar curve and would steepen sharply at about the same time: two Englishmen quickly making good, fast, in America. Pegg explains it as "people with a similar drive, similar interests, similar impulses, being drawn together for that very reason".

Does he feel, then, that he was always destined for fame?

"I sort of blithely walk through everything. When I started doing stand-up, I didn't have a five year-plan. Eddie Izzard famously had a five year-plan, of what he wanted to do, what he wanted to get out of it. I didn't, I just took it as it came. When we were getting Shaun ready I didn't worry that it wouldn't ever get made, I just assumed it would. Whether that was optimism or naivety, I don't know. Probably the latter."

He is comfortable enough, for a time, to talk about a few more of his showbiz pals. David Walliams is roped into the quantum attraction, you-meet-the-people-who-are-going-to-better-you theory ("It was very obvious to me that David was going to be a successful comedian when I knew him at university, and if anybody was going to get anywhere it was gonna be David"). And he enjoys a good giggle on relating the news that Martin Freeman has been cast in the lead role in The Hobbit, the prospect of his friend having to endure the sci-fi convention circuit absolutely delighting him. "Martin's the anti-me: a soul aficionado and a vinyl junkie – absolutely not a resident of the geek universe. Not the type of person who will relish the attention he'll get for being Bilbo Baggins. Ha!"

The interview draws to a close. We talk a bit about time travel, then about The X Factor. I feel that I've got to know him a little bit, admittedly aided by the lubricant of his memoirs and his commitment to publicise it. The Schnauzer hasn't even had a look-in. Curious, after all, I ask about Minnie. She's doing well, says Pegg. "She still likes eating socks."