"People still yell 'Dan' at me!"

“He physically abused me every day,” says Stephen Mangan of recent co-star Matt Le Blanc. “He’d pin me down and fart in my face.” We think he’s joking. But the lank-haired comic actor has such a scurrilous sense of humour that it’s hard to distinguish between a deadpan gag and a world-breaking exclusive.

It’s no wonder, then, that he’s been chosen to breathe life into Douglas Adams’ huckster detective Dirk Gently in a new BBC drama...

Have you had to contend with any irate Douglas Adams fans since being cast as Gently?

I’ve stayed away from the message boards but I have been contacted on Twitter by a few fans. So far they’ve been really nice. They’re very passionate and protective, plus when a character has only existed in book-form everyone has their own idea of what he should look like. In fact, I think Dirk’s described as quite chubby in the book, so I’ve fallen at the first hurdle. I should have gone method like Robert De Niro and eaten 40 burgers every morning.

Isn’t Gently the sort of antihero in which you seem to specialise, though?

I think I do have a knack for playing b*stards. Which unfortunately means I do end up playing a lot of them. But there’s an innocence about Dirk. He may be a bit of a conman but he’s got an enthusiasm and wonder to him that’s more appealing.

Do you still get people screaming “Dan!” at you because of your I’m Alan Partridge appearance?

I do. It’s faded now to probably only once a week [laughs]. But there was a time, especially just after it came out, when I couldn’t go out without it happening. I’d have to stand there at the till in a record shop for about five minutes while some bloke shouted “Dan! Dan! Dan!” at me. The joke only works if you say it several times, so I have to stand there and laugh politely then go, “Can I just pay for my Barbra Streisand CD now, please?” There’s no question that it’ll follow me for my whole life. And why not? It’s one of the things I’m most proud of.

In recent mockumentary Beyond The Pole, your co-star Rhys Thomas had to spit in your mouth. Was it done for real?

It was and I can honestly say that lying down at minus 30 degrees, with a man standing next to us holding a shotgun in case we were attacked by polar bears, while Rhys Thomas hawked every bit of phlegm from every corner of his body and spat in my mouth is one of the worst things I’ve ever done. We probably did it about eight or nine times so there was no lunch for me that day.

You’re also well known for the Barclaycard adverts. What’s the strangest ad you’ve been offered?

The weirdest one I ever did was one of my first advert auditions. I went up to an attic in Soho and it was just one middle-aged man and a camera on a tripod and he said, “Look into the camera and say your name, then turn to your left and turn to your right.” Then he went, “Now can you take off your top and growl like a tiger?” I thought, “Oh God, here we go, the casting couch,” but actually I got the job and it was an Estonian chewing-gum advert that filmed in Cape Town. So a week later I found myself in a Jacuzzi with eight women in bikinis while I was wearing the world’s smallest thong. Which had a tiger’s face on it, by the way. The entire thing revolved around me putting chewing gum in my mouth, being caressed by lots of women then going, “Grrr” to camera before it zoomed in on my crotch.

Is it on YouTube?

I bloody hope it isn’t.

Dirk Gently is on BBC Four on Thursday 16 December at 9pm

(Image: Rex Features)