Smash hit

’ ” He took himself to Blackpool and started a course in musical theatre at a college in genteel Lytham St Anne’s. “I thought that was really posh. Some of the people didn’t understand a word I said. It was a great laugh. I just got drunk and lost my virginity and started up a band and had a really good time.” The students did West Side Story and Guys and Dolls in the Grand Theatre. Simm had leading roles, but he was already drawn to straight theatre and joined an amateur dramatic society, doing Amadeus and Billy Liar.

London beckoned, and a place at the Drama Centre in Chalk Farm, where he overlapped with Anastasia Hille, Helen McCrory and Tara FitzGerald. “It was a long time ago now.”

I point out that he looks young for his age. “Oh great!” he says, genuinely pleased. “Because I got an injury to my knee and I’ve spent most of the year learning to walk again. It was first smashed up by a stuntman ten years ago. I was playing a killer, as I was wont to do in those days, in a thing called Chiller. At the end I was running up a moor in Yorkshire and these two policemen came into me like a freight train; my foot got stuck in a clump of grass and my knee just went crack. Nightmare. It took six months to get over that, and my career was just taking off. Recently, when I came back from filming Sex Traffic in Romania, it started to come out of joint when I was walking down stairs. So I thought, OK, I’ll put time aside and have the operation. Because of that I’ve had to go to the gym and do physio, so I’ve never been fitter.”

He relishes his new clout in the industry, but has no interest in doing long-running serials: “I like films, and the dramas I do are like films. If you do good drama, from the producer down to the director, cast and writer, it’s wonderful.”

So when David Yates, the director of State of Play, asked him to do Sex Traffic a week after he had returned from Tunisia, where he was playing Caligula in “this huge epic thing” called Nero, he jumped at the chance. The two experiences couldn’t have been more different. Nero, he says, was a wonderful opportunity to play “the most evil man that ever walked the Earth”, and he “camped it up to high heaven”. In Sex Traffic, by contrast, Simm acts down, not up.

“I thought I knew about that situation, but obviously I didn’t, because when I read it I was shocked. It’s the only thing I’ve done where I thought, ‘This needs to be made.’ ” The film is riddled with vile cruelty, most of it calculated, some almost casual. Simm found the finished drama horrifying. “There were certain things I couldn’t watch. I knew what these bastards do to women, but I didn’t realise the scale of it.”

The short turnaround left little time for research, but he trusted the writer, Abi Morgan. “It’s all there in Abi’s script,” he says. “Not for one minute did I think any of this was embellished or made to look worse than it was.”

Later, Morgan will confirm this to me, insisting that she left out several true episodes she had researched that were even more upsetting than the material in the drama.

Daniel Appleton is not a huge part, but he is the thread that draws together this complex thriller. Simm’s main concern as an actor was to differentiate his role from Cal McCaffrey in State of Play. “I based him on one or two people I know and put glasses on him. I wanted him to be quite small and a bit geeky and quiet. I hope that comes across.” It does.

But the working-class rocker lives on, too. He wants to make a comedy and is thinking of trying his luck in America. If he continues to choose his work as shrewdly, it will always be worth watching. Small or not, John Simm is a big presence.

Sex Traffic, Thursday, Channel 4, 9pm