The original Simm

Simm is widely recognised as one of the finest actors of his generation, whose presence in TV credits itself lends a kind of class and adult seriousness. This is partly because he has chosen his telly, and its writers, so well: from The Lakes (Jimmy McGovern) in 1997; to Crime and Punishment (Tony Marchant); State of Play (Paul Abbott), Sex Traffic (Abi Morgan), and now Dr Who (Russell T Davies). But it's for Life on Mars, the 1970s cop show with an X-Files-type twist in which he plays DI Sam Tyler, the 21st-century detective lost in a world of unreconstructed blokes and downtrodden birds, that he's now most famous.

Having been a cult-ish actor, most often employed in heavyweight one-off dramas, he's now as mainstream as a soap star - and as frequently recognised. Primary school kids are one thing; obsessive adult fans quite another. As we leave the theatre, a complete stranger points at him and shouts at the top of his voice: "I was watching that man on telly last night!" Simm blanches and swears beneath his breath.

"I've reached a zenith of it with Life on Mars. It's driving me crazy. There were people queuing at The Bush to get me to sign DVD box sets, and I probably gave them fairly short shrift. I don't open fan mail anymore. I've got piles of it, which is not a boast, but a fact, and I just can't be bothered with it. The whole thing becomes a bit narcissistic after a while. I've run out of photos and I'd be there for months reading it, so I just don't look."

His agency filters the mail for "nutters" (and knickers: Life on Mars also confirmed Simm's sex-symbol status) but occasionally they get through and scare him a little. He and his wife went to Glastonbury this year, "for a bit of a blow out" without the children, but Simm had to keep scuttling backstage to avoid the shouts of recognition, and then the hordes of people swooping on him.

He confesses to a growing streak of misanthropy as he grows older - one of the problems with Simm as an actor is that, save the greying hair, he never looks much older than 25 - and, it's true, he does sound quite Meldrewish. "I got really really angry with people falling asleep at The Bush," he says. "I know it was hot in April, and I know it's a very small theatre, but if you're old and you're tired, don't sit in the f***ing front row. Don't come to the theatre."

The success of Life on Mars came as a shock. "I had an inkling, because the production company, Kudos [maker of Spooks] had a good track record, but I didn't think for a minute it was going to be massive. I didn't even know if it would work. Let's face it, the premise is ridiculous!

"The challenge was obviously to make it believable, so I told them the only way I'd do it was to approach it as I approach everything, which is with absolute realism. I couldn't just wake up in the 1970s and say, 'OK, let's get on with solving crime!' It had to be an absolute nightmare for Sam Tyler, particularly the first couple of episodes. It was a really hard job, the hardest thing I've done by miles."

There won't be a third series, he confirms, though his co-star Philip Glenister is now filming a spin-off show, Ashes to Ashes, which follows Gene Hunt into the 1980s. Simm has made no secret of his anger at the BBC's inexplicable decision to cut the budget for the second series, and of his eventual annoyance at playing the nerdy straight guy, the continual butt of Gene Hunt's razor-sharp one-liners.

But it was the strain of filming for six months at a stretch that convinced him not to sign on the dotted line. "I just couldn't bear to be away for another half a year. Life came banging through the door: I just thought: 'Wait a minute, you're going to miss seeing your children grow up for three years here.' And I wasn't about to do that - actually, I don't know if I'd have had a marriage left if I'd carried on. It was a single man's part, Sam Tyler."

Simm grew up in a council home in Nelson, a fairly grim mill town near Burnley, Lancashire, and for five years of his childhood his parents were separated. Stability and normality for his kids - with the exception of that star turn as The Master - are clearly pretty important to him.

He was once a well-known party animal: he describes the club scene in Manchester in the late 1980s, and the Britpop era in and around Camden in the mid-1990s, as "two of the greatest times of my life", but nowadays he goes to the pub with mates to watch football, or he parties at home.

Rumours of a second series of State of Play, Paul Abbott's brilliant 2003 political drama in which Simm played Cal McCaffrey, chief reporter on a fictional Sunday paper, are sadly untrue. "God, I'd do that if they rang me tomorrow," he says. "I've read about it with everyone else, but I haven't heard anything, so I have to say it's probably bollocks."

But Hollywood is making a film version, set in Washington, in which Bill Nighy will reprise his role as the paper's editor and Brad Pitt will play McCaffrey. Simm screws his face up. "I understand they need big names, and it's America, but, you know, it does piss me off a bit. It'll be a big glossy thing, nothing like what we did. To get on in America you've got to play the game, and I'm 37 and just not up for it. They know who I am, and they know where I am. I'm not going to go over there and start begging for work."

Not that he needs more work, of course. The success of Elling, and his current ubiquity on TV, have made him very hot property indeed. But if you see him in a pub in Crouch End, honestly, don't try to tell him that.

Elling is at Trafalgar Studios (0870 060 6632) until 6 October.