It was very much an accident, but I possibly ruined Eddie Pepitone's day a few weeks back, if there's any truth in that old Morrissey lyric about us hating it when our friends become successful (and let's face it, there is).

During a transatlantic phone chat previewing the Brooklyn-born comic's upcoming London dates I mentioned the UK success of his old contemporary Louis CK, and his mammoth, much-discussed O2 show. Pepitone, blissfully unaware, was momentarily speechless. "Oh shit, you're kidding me," responded the comic, eventually. "But, yeah, he's amazing. It's wild to watch how that happens."

In fairness, it's finally beginning to happen for Pepitone too. The 54 year-old has long been a cult figure in the states, chiefly via stand-up stages but also handy roles in shows like Flight of the Conchords (where he played the sign guy). Now a documentary about his life, The Bitter Buddha, is bringing his rant-filled work to a wider audience over there, as the international gigs rack up too.

His debut Edinburgh show, Eddie Pepitone's Bloodbath, was one of the most talked-about at last year's Fringe, such a unique display of furious self-loathing that I was surprised to be greeted by such a softly-spoken chap when he answered the phone. So can he keep conjuring up the required rage during this lengthy London run?

You've a hefty stint in London coming up. These three-week runs must still be fairly novel for you?

I know, I'm excited and scared, the length of it. I did Edinburgh last year and a lot of people liked the show, and I've got a few gigs from that in the UK. But this is new for me, all this overseas stuff.

How did it happen?

I don't know if you'll know a guy called Paul Provenza [indeed, we interviewed him last year]. Paul was pushing me to do Edinburgh, he was saying 'oh my God' - the way my act is, the unhinged American - 'they're gonna love you.' He really pushed me and I needed a push because I'm a bit provincial, I like to stay at home. But I'm really glad I did it and I was amazed at how great the audiences were.

Your act is pretty unique...

I'm not sure exactly what I have except my angst. I really try to connect emotionally on stage. Of course I want to be intellectual and I think I am, but I really want to express this deep-seated dissatisfaction I have with everything, and I kind of try to take my rage to absurd levels.

Do your regular rage-targets still work in Britain?

They did in Edinburgh. I think dissatisfaction and frustration, trying to live the life you want to live, is universal. We seem universally to battle the same demons, getting to our truth and happiness, whatever that is - it seems to be elusive.

You're enjoying unprecedented success now though - isn't anger harder to come by?

I'm not having that problem because I also find that success brings its own set of frustrating things. You get busier, and being busy everybody wants a piece of you, you have to be in a lot of places, travelling a lot, and that makes me angry. I'm like a comfort guy - I realised that success makes you have to run your ass off, doing this, doing that. And also as you get success, you get addicted to it a little bit. I think we live in this instant gratification world, the internet has made us like that, so if I don't have the attention that I had yesterday I'm angry.

Might this long spell in London exacerbate that? A lot of comics go mad during Edinburgh.

You know what, I almost lost my mind in Edinburgh. Seriously, because it was four weeks, and this is three weeks, so I feel like I went through it already. I do get homesick, I like to be around my cats, I'm very domesticated these days.

A lot of people who've seen your shows would probably be quite amazed about that!

Exactly, they just think I'm this angry lunatic who has no tether to any kind of reality.

So when you turn up for an acting role, do people think you'll be hard to deal with?

Yeah, but then people are always kind of disarmed by the fact that I really get a lot of my rage out on stage, and offstage I'm very 'hey, how are you?' - I tend to be quiet. Even though I do get annoyed. The genesis, the source of my rage is always kind of with me, I'm not sure where that comes from, and why I get so upset. I get upset at little things. In LA it's traffic.

If you need any fury in London you should just hire a car.

I drove in Scotland and I almost had a heart attack. Everything is reversed!

Having seen other US acts over here I'd imagine your set is pretty nailed down now?

I have a lot of pieces that I love to do, but I also love to give myself room for improv. I'm having the most fun doing stand-up when I'm in the moment, just riffing with the crowd and with my thoughts. Sometimes my act seems a little disjointed, I like to do it like jazz, where I'm just following a rollercoaster of my mind really, and I'm just trusting that it's funny, and interesting - that's the job of the stand-up, to be interesting too.

How did you get into comedy initially?

I studied acting for a bunch of years, but I was in acting classes and I would do scenes from dramas and everybody would be laughing, and I'd be like 'what the hell's going on?' My parents and their dysfunctional marriage really made me into a comedian though, I'm like that typical clichéd story of a kid who grew up just being funny to survive, my sense of humour was my saviour.

I started making people laugh in earnest when I was about 11 or 12, maybe 14 when I really knew I was going to be a comedian. When people started laughing at my dramatic work I knew comedy was the way I was going to go. But I loved drama, I loved acting, and I try to incorporate that into my stand-up. I really think my stand-up has a theatricality to it. I'm not just saying words.

You're from Brooklyn, but now you live in LA: how did that come about?

I got a part in a movie called Old School, with Will Ferrell. One of the writers of the movie thought I'd be good, so I went to LA and I kind of fell in love with the beach and the sun. It was just time for a change. I was 40 and I'd spent my whole life in New York.

So what's been your biggest acting role so far?

You know Conan O'Brien? He has a talk show here and I've become the heckler in the audience, these written bits where Conan will be in the middle of saying something and I'll be in the audience and I'll just get into him. Heckling has become a big part of my act. I do this closing number [in my show] where I heckle myself. Before that I had a recurring role on Sarah Silverman's show and a really nice part on Flight of the Conchords. I'm hoping that the movie expands my popularity so that I can get juicier roles, and I'd love to get my own thing. Right now I'm pitching bigger shows.

You're amazingly calm offstage - would you be a mess without stand-up?

(Laughs) I would definitely have to find something else. Like woodworking, like Jesus did - he was into carpentry before he found the prophet thing. If I don't perform for a week I start getting squirrely, a little weird, like something builds up inside me. If I didn't have stand-up I don't know what the hell I would've done. It really has been a way to just express myself and build a life.

It must be good to know, whenever you're stressed, that you can go and shout at people that evening?

Exactly. I can just rip into it and get away with it.

'Eddie Pepitone: Electrified' is at the Soho Theatre from the 6th to 25th of May. Visit www.sohotheatre.com for details.