Shortly to be reunited onscreen in The World's End, and with a host of solo TV and film projects in the works, the comic boy-man duo seem to be going from strength to strength

What a pleasure, what a sugar rush, giddy-kipper joy it is to watch Simon Pegg and Nick Frost together in a film by Edgar Wright. I went to see a preview of The World's End, the final part of the Pegg/Frost/Wright "three flavours Cornetto" trilogy, and I laughed and laughed for three-quarters of an hour. Then I stopped, as we were only allowed to see the first 45 minutes of the film, for some reason. Either the special effects people were still cranking out the CGI, or the last hour is REALLY BAD (not so, according to those who have now seen it).

Anyway, the part I saw zipped by, a slick, high-spirited blast of jokes and small-town action, a particularly English setup (a group of old friends go on a pub crawl) suddenly slammed into intergalactic sci-fi big time when our heroes get into a fight with a group of replicants. The replicants appear to be made of the same hard-soft plastic as small boys' action figures. When their heads pop off, they bleed blue: a reference, Wright has said, to the way that schoolchildren's hands are always stained blue with ink.

The Cornetto films – Shaun of the Dead, Hot Fuzz and now The World's End; all films written by Pegg and Wright, starring Pegg and Frost, directed by Wright – abound with references to pop culture, contemporary life, and, especially, other films. Shaun of the Dead was a homage to zombie flicks, particularly Dawn of the Dead; Hot Fuzz was a buddy cop film; The World's End is sci-fi invasion à la Day of the Triffids. Fans of the films, which celebrate their chosen genres rather than mock them, glory in spotting references such as "Landis Supermarket" in Shaun. There are, no doubt, umpteen movie in-jokes in The World's End, though I wasn't geeky enough to spot them, other than the crashing-into-a-fence gag that features in Shaun and Hot Fuzz.

The films are designed for upbeat fun, full of laffs and explosions and mickey-taking boysy camaraderie. They assume a particular mindset in an audience, a cheerful city slacker attitude. And so Wright, Frost and especially Pegg have become the British poster boys for good-time geeks, boy-men who love space films, shoot-em-ups, indie music and comedy, and who see no reason to change their taste as they get older. That was Pegg's character in Spaced and Shaun, and that person, for his fans, plays in the background of all his characters, no matter how varied. Which works when he's acting in Mission: Impossible and Star Trek, less so when he's playing other, more conventional parts.

Anyway, for those of us who found the last Star Trek as enjoyable as being swallowed by a million-mile-wide arcade machine, The World's End is a welcome reminder of why we like to watch Pegg act in the first place. The story centres on Gary King, once the coolest kid in sixth form, now a sad-sack 40-year-old dropout still living in his home town, Newton Haven; still living in the past. This is Pegg's role and it's interesting to see him play it: Gary looks rough and is thoroughly irritating, one of those tedious party-time-all-the-time bullies who insists that everyone must get drunk to screw The Man. Though he's likable – Pegg is always likable on screen – Gary is also pathetic and a bore. He plays Primal Scream's Loaded like it's a manifesto for life. Frost is Andy, one of four friends that Gary entices/browbeats out of their conventional, moderate lives back to Newton Haven for a pub crawl, a recreation of a memorable night in their 20s when they didn't actually make it around all of the town's drinking establishments, but they were happy. Or, at least, Gary was.

The idea for The World's End came from Wright, who wrote a screenplay about a wild pub crawl when he was 21. After Superbad, he realised he couldn't make that film: but he could make it the kick-off for a different, more adult movie adventure. He and Pegg wrote the script; Frost read it, made detailed notes, and all three of them went through it line by line until it was right.

Which means that Pegg and Frost, at least, know what happens in the second half of the film. So, let's ask them. How does it end up?

"Everything ends up in a gigantic argument," says Pegg, "about the relative merits of the stupidity of humanity. And why being stupid, and being allowed to be stupid, is a good thing."

Neither Pegg nor Frost is stupid, though they certainly allow themselves to be; like most clever, funny people they love silly jokes. Lines in the film include Andy saying: "I haven't drunk for 16 years." Gary: "You must be really thirsty." In real life, they're quick with ripostes, and slide in and out of ridiculous characters to make each other laugh. Frost can be quite breathtakingly filthy and dark: his everyday jokes are unrepeatable in a respectable Sunday newspaper. They might stain your soul.

The Cornetto triology films: Shaun of the Dead; Hot Fuzz; and The World's End

We are in unstained Claridge's, in a lovely suite, with a balcony and vast shiny bathroom. Pegg and Frost lounge on the sofa; I sit opposite them, on the other side of a coffee table, like I'm the TV. This reminds me of when we first met, in the late 1990s: they shared a place with my then-boyfriend, a big, untidy, grotty flat in Highgate. The TV was never off back then, not even when music was playing, or people were around, or everyone was in bed. The flat was full of toy weapons; the fridge was empty; the coffee table groaned under the weight of all the fag ends, beer cans and lads' mags. At least once a week, everyone sat in the pub all day.

Despite slacker appearances, however, Pegg was a driven man, and out of this seemingly slapdash life he created Spaced, the cult TV sitcom which told the tale of Tim (Pegg) and Daisy (Jessica Hynes, who co-wrote Spaced with him), who pretend to be a couple in order to get a flat. He brought in Wright as the director, and Wright's vivid, cartoonish style, his cuts and whips and crashes, transformed Spaced's story – of small people living small lives – into an epic adventure. The point being, of course, that a small life is still significant and thrilling to the person living it, whether or not it's accessorised with zombies/replicants/murderous members of a Neighbourhood Watch committee.

Spaced and all of the Cornetto films explore this. They also explore the idea of staying young and foolish in a world that wants you to knuckle down and take life seriously.

"All the films have been about conformity," says Pegg, "about whether you are conforming to the zombies, or whether you are conforming to the NWA (Neighbourhood Watch Association), or whether you are conforming to the Network. It's all the struggle of the individual against conformity. In World's End, Gary's friends have all conformed, succumbed, they have all become…"

Frost: "Robots."

Pegg: "Robots. But Gary is not a hero for not conforming; he is a tragedy for not conforming. The film is a lot darker than anything we've done before in that respect, and it doesn't offer the answer. In the film, there is no middle ground. There's no happiness and success, there's no: 'I'm content.' Though in real life, Nick and I have reached a point where we're doing what we enjoy."

Frost: "We've gone under the radar. We're adults, but we get to do what we want. I mean, look at my T-shirt!" (It says "Happiness Is a Warm Gun." He is 41.)

The myth about Pegg and Frost is that their lives have stayed the same, that, since Spaced and Shaun, they've remained stuck in front of the TV, smoking weed and sharing a bed (when they met, Frost had nowhere to live, so he came to sleep on Pegg's floor and ended up co-opting his duvet). But no one's life is static, particularly if you suddenly hit the big time. Over the past few years, Pegg and Frost have popped up in various high-status projects. À deux as the Thompson twins in Spielberg's The Adventures of Tintin: The Secret of the Unicorn and in Paul, an American roadtrip-turned-alien-kidnap movie which they wrote together. But also separately in two Mission: Impossibles, two Star Treks, plus Burke and Hare, Run Fatboy Run (Pegg); and The Boat That Rocked and Snow White and the Huntsman (Frost).

They move away from each other and return. They love each other dearly, but they have other people to love now. Pegg is married, with a daughter, and lives in a country house in Hertfordshire; Frost is in south-west London, with his wife and small son. Their lives have opened up. And The World's End reflects this, with a larger cast of central characters: Martin Freeman, Paddy Considine and Eddie Marsan making up the fivesome.

There are other changes. One of the interesting aspects to The World's End is that a few years ago, Pegg gave up alcohol. In fact, of the five main actors, two don't drink (Pegg and Marsan), one barely does (Freeman) and two (Frost and Considine) have cut down massively. "No one's a beer lord," says Pegg. Despite this, given the film's premise, Pegg and Frost fans are constantly tweeting about how they're going to go on a World's End-inspired pub crawl, or how they'd love to go for a bevvy with their heroes.

"I think, no, you wouldn't," says Frost. "It would be OK for 20 minutes, until you wanted to get a shot with the next pint. I like to go home early, that's my thing. My idea of a pub crawl lasts from midday until 5pm, then I can go home, play with my kid, have tea and go to bed."

Pegg says it was a bit weird for him to spend so much time sitting in pubs for the film: "I got a sort of Pavlovic sense of dread. I like not going to pubs now. I relish it."

We talk for a bit about the pub as a concept: in Shaun of the Dead, it's the heroes' fortress against the zombies; it's central to all of the Cornetto films.

"Well, it's central to British society," says Pegg. "It's so important. But when you strip away the social aspect, then it's just somewhere that you drink alcohol. A lot of our society is based around emotionally anaesthetising ourselves. You can call it a social lubricant, but there is so much alcohol consumed in this country, and I wonder if that's related to how emotionally repressed we are as people? Do we drink because it helps us let go, or not face that level of repression, or whatever?"

"It's because it's the only control we have over our lives," says Frost.

Frost still enjoys a drink: he's an amazing cook, proper chef standard, and good food and wine is important to him. Still, now he's a parent, he says he has a different take.

"I am kind of amazed just how much parents drink to null the pain of being a parent," he says. "You often go somewhere with a bunch of other parents and they're hammering the wine. Also, in that two-hour period between baby's bed and your bed: let's get as much wine in as we can because it's 'our time' now. As a worrier, I have an issue with that."

Pegg: "What if the baby wakes up and drinks bleach?"

Frost: "Yes! What if I drink four Stellas now, and I have to drive to the hospital? How do I do it? Do I have to wait for a cab? Or, if I phone an ambulance, and they see I'm pissed, maybe they will take him away from me."

Pegg: "That's why it's easier just to not drink, I think."

Frost: "Or not have kids."

When Frost was in his early 20s, he gave up drink for three and a half years. He'd been living on a kibbutz in Israel, drinking a bottle of vodka a day. He ended up weeing in his bed while his girlfriend was in there too: "She had to kick me really hard in the head for a long time to wake me up, and I thought: 'No more.'" So he came back to London and stopped drinking.

Not many of his friends accepted it; a girlfriend left him because of it. After a while, though, he thought he'd have another go – and his first drink was in the World's End pub (no relation, oddly), in Camden. He phoned up Pegg after four pints in a complete state, and Pegg came and got drunk with him: they ended up walking home to Kentish Town with their trousers around their ankles and finished off a bottle of Scotch.

Now Frost has a theory about drink and drugs: he thinks they have an inbuilt time limit for everyone and you have to listen to yourself to know when you have had enough. Neither he nor Pegg smoke cannabis any more because it doesn't agree with them. And also, they're really busy: dealing with hangovers isn't really an option. "No," says Frost. "I'm writing my will tomorrow." He's not joking.

Next up, for Frost, is a dance film, Cuban Fury, which comes out next January. He stars alongside Quincy Jones's daughter Rashida and the idea came from him, sparked by what he calls "that look that thin people do to fat men when they dance quite well, that 'oh, he's pretty good for a big guy!' look. I hate it." He danced for seven hours a day for seven months to get up to the right standard for the film, got over the humiliation of being taught by buff Brazilian hunks in a fully mirrored room until he became a proper toe-twirler. ("He can't do Strictly Come Dancing now," says Pegg. "He's too good.") There's also a Sky sitcom, Mr Sloane, written with Bob Weide, who worked on Curb Your Enthusiasm. Frost worked very hard on that too: up at 5am working on the script, filming all day, then home, tea and more scriptwork for two hours on top of that, plus learning lines at weekends. Frost doesn't find everything about his job very easy. He'd like to direct: next year, he's hoping to direct a couple of TV episodes and work up from there.

Simon has a couple of new projects post-World's End: he's just finished filming Hector and the Search for Happiness, which is based on a novel by François Lelord. He got that job when he was doing a guest spot in a Frank Darabont pilot, Lost Angels, in LA; there was a launch dinner, and he sat next to the casting agents, who said he should read for Hector, so he met the director, Peter Chelsom, and "we got on really well and we kind of cemented that I would do it on that first night". This is very Pegg: he works hard, but he has luck, too. Notoriously, after Shaun of the Dead, he said "It's not as though I'm going to star in Mission: Impossible 3"; and then JJ Abrams asked him to do just that. He can seem to charm projects from the skies.

Still, as much as his job is going well (he's doing a thriller in Australia, Kill Me Three Times, in September), he's worried about the impact on his life: his daughter is four now, and he and his wife think she needs consistency, which means not dragging her out of school every few months to hang out with Daddy on the other side of the world. "I might have to have a rethink," he ponders. "I might have to change my career plan and work at home in television dramas or something. I would love to be able to do something that kept me at home. World's End was great because we were filming within half an hour of where I lived. Elstree was 15 minutes from my house, Letchworth, half an hour, and I was sleeping in my own bed."

"The thing is," says Frost, "we love working. I get really anxious about it, and beat myself up, but I love it. I'd be happy doing anything on a film set. Also, you have to work. You'll be forgotten if you don't."

What they want, of course, is a good time all the time: a fantastic, creative, world-beating job, and a stable, happy home life. To live like a teenager and an adult. The World's End says they can't do it; to me, it looks as though they can, and they are.

The World's End will be in cinemas from 19 July

• This article was amended on 7 July 2013 to remove spoilers.