'Seann's a f*** up!

F***ing look at him! But he'll make it in the end!" As a pitch for your client's sitcom, it's certainly bold. The late, legendary and combative agent Addison Cresswell - who famously negotiated Jonathan Ross's £18m, three-year deal with the BBC and oversaw the careers of Michael McIntyre, Kevin Bridges, Lee Evans, Jack Dee, Alan Carr and Dara O Briain among others - understood something about Seann Walsh's talent. Relishing Cresswell's Cockney vowels in his mouth as he recalls that fateful meeting with television commissioners, Walsh remembers his bewilderment and their laughter at the backhanded compliment that's since become his mantra.

The 27-year-old has lately resumed performing his Edinburgh Comedy Award-nominated stand-up hour, The Lie-In King, for the first time since the Fringe, labouring to summon routines that were hastily pulled together in three months and now remain undocumented outside his own head. While emphasising Walsh's gifts as an observational and physical comic, the show also features cheeky impressions of McIntyre (right), Bridges and Josh Widdicombe. And yet, with his leonine hair and willingness to acknowledge his mistakes, Walsh retains an identity that's (almost) entirely his own.

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For example: too lazy to approve his recent DVD's cover art, his management "gave me a tail, which was quite embarrassing. It nearly made me cry at one point." And, hours after we meet, he's due to be recording a radio pilot with his friend and support act Romesh Ranganathan. Walsh reckons the other stand-ups auditioning for the slot will have prepared exhaustively, but not him: he's just hoping to wing it and ask Ranganathan daft questions.

We're chatting in a bar in London's Leicester Square where Walsh, who struggled to find material for his Royal Variety Performance last year that didn't include visiting the off licence or feeling hungover, is casually sipping tea with a side cup of Smarties. "I had to go back three or four years," he smiles wanly, "which made me realise that all I've written about over the last two is getting drunk and living a bit of a s*** life."

He received the potentially career-changing call to meet Prince Charles while in his bedroom. But by the time he'd got to the living room, he'd forgotten all about it. "I had no idea how big it was. It was only later, when it was announced and my phone went crazy, I had a look on the internet and realised that it was what made McIntyre, when he first did the Man Drawer routine. Micky Flanagan too."

Confessing that he barely watched Seann Walsh World, his online clip show on Comedy Central - "just go on the internet, why do you need to watch a show about it?" - he'd returned to the Fringe "off the back of things that were questionable, so it was probably quite important that the show was decent". With an ambitious finale, involving Ranganathan and another comic, Henry Paker, joining him on stage to refer back to previous gags, it triumphed to various degrees on any given night. He remains justifiably proud of the show, not least because he was too hungover to try out the ending beforehand, his Edinburgh run launching with him improvising it with his mates.

A stand-up of seven years standing who grew up idolising and impersonating Jim Carrey, Walsh left school with a single GCSE in drama and worked in a pub. Even after following the likes of Jimmy Carr, Shappi Khorsandi and Hal Cruttenden by taking Jill Edwards's industry-respected comedy course in Brighton, he faltered when speaking to audiences because "I wasn't a very cultured person, I was so ignorant of the world". After honing his craft on Brighton's supportive scene, "where all the comics watched each other and talked comedy the whole time", he started gigging in London "and found it very different, the aggression here … what a bunch of horrible people!"

The Lie-In King is essentially about how he seemed to have it all going for him - appearing on Channel 4's topical stand-up showcase Stand Up For The Week and making his first venture into sitcom with Big Bad World on Comedy Central, playing the feckless, bedsit philosopher Eggman - while despairing about living in a "crack den basement flat".

"When I was younger, if someone had been on telly, you'd assume they were living the life of Riley, turning up at places in limos," he reflects. "But even though I've had my name in titles, I really did live in this horrible, disgusting place. On my own, trying to push myself forward as a person. And it just went really wrong, there were too many problems. I wanted the show to be about how I'm such a f***-up."

He hazily recalls his first, callow appearance on Mock The Week, "when I barely had 20 minutes of material. If someone had heckled me at a gig or dropped a glass, I would have just frozen. So doing that show was massive. I didn't have any money. I think I'd done one paid weekend at the Glee Club in Cardiff. I was sleeping on Paker's floor and … no, I didn't even know Henry then! I was in Edinburgh and they flew me down. I was just a drunk 22-year-old doing Mock The Week. Mental!"

Like plenty of his peers, Walsh is "absolutely terrified" of waking up to discover that he's no longer funny. Yet intriguingly, his insecurity relates to his growing success, with his "messy" efforts to revive The Lie-In King caused in part by the fact that "my life's changed quite a bit".

After moving to a nice part of Shepherd's Bush in October, he sees Bill Bailey around, "though he wouldn't recognise me", and now lives with his girlfriend, the actress Rebecca Humphries, whom he met on Big Bad World. They've acquired their "own cupboards, mine a working-class one full of tins, hers full of wheatgrass, herbs and spices. So I'm learning a new way of life. I'm becoming the guy who's got to be disciplined and behave himself. I'm not allowed to stay out indefinitely. I probably was naughtier than most, so hopefully, me being called home is funnier than most blokes getting it."

Even so, he clings to his younger, more shambolic self. "If I've got to write, I feel like I turn it on: 'I'm a comedian today'," he explains. "I wake up and go into the persona. I'll be on the Tube and become the person that people pay to see. I get a bit grumpy, dishevelled, lazy and will purposely get a bit more clueless about life. I don't like being wiser, I like that ignorance and sometimes I have to turn it up. I have to remind myself: 'you're an idiot'."

He appears momentarily wistful. "Because one day I'll go on stage and realise I'm not him. I'll have a girlfriend, kids, and no idea who I am."

Indeed, when he shared his and Paker's idea for a sitcom with the BBC, set behind the scenes of a television show, it was rejected for not showcasing enough of his stand-up personality. They subsequently decamped to their writing cottage and came back with an idea that better reflects his reputation. "I really think people will like it," he enthuses. "It's all I think about at the moment." Every day, he wakes up wondering "what can I do to help me get this made?"

A significant step could be his first lead role, in the BBC One sitcom pilot Monks, which he shot in January. More than 14 years in the making, first as a one-off Radio 2 show in 2000 (as Hey Hey We're The Monks, with Bill Bailey and Catherine Tate) then in 2008 (as an unbroadcast television pilot with James Corden), Walsh is the latest prospective incarnation of Gary Woodcroft, a benefits cheat who joins a monastery to escape prosecution.

"I suppose there are similarities between me and Gary," he admits. "He's lazy, he avoided paying tax." He hesitates. "What I mean is, if I wasn't a comedian, I really have no idea what I'd be doing now. I probably would be on the dole. I wouldn't be bothered. I'd be Gary. Maybe I'm not even acting, maybe they just thought 'Seann's quite like Gary isn't he?'"

He was concerned about appearing alongside such established comic actors as James Fleet, Mark Heap and Justin Edwards, "who can do drunk even better than I can". But it sounds as if the producers knew what they had.

"For the credits sequence I had to walk on a treadmill in front of a green screen. And I got really self-conscious. Walking? How do you walk again? It's the little things that always trip me up the most."

Seann Walsh plays The Garage, Glasgow on March 22, as part of the Glasgow International Comedy Festival (www.glasgowcomedyfestival.com), and will return to the Edinburgh Festival Fringe this summer