Mark Heap says he'd like to murder people who litter... or at least shoot them with a crossbow.

How would you describe your character?

A loner. He's on his own, at the back of the bus, and doesn't say a great deal, just looks sort of angry. He thinks the author is a total oaf and an idiot, and hates everyone else pretty much. So it's very unsociable for me, because I'm just sitting on my own, I haven't got any little chum to play with during filming.

How would you describe the tone of it? Is it a whodunnit, an out-and-out silly comedy, a sort of pastiche?

It's all of those, I would say, probably. It's a sort of romp. It's sort of Murder On The Orient Express but taken up a notch maybe, slightly sillier. But still, hopefully intriguing.

The cast is fabulous.

Some of them, yes. Some of them are unpleasant.

Did you have a good time filming in Blackpool?

Yes, it's nice to spend time together rather than go, 'Goodbye' and go home when you're filming in London. It's much more sociable. It was controlled sociable, we didn't go off the rails.

There are some excellent murders in this. What do you think would be the perfect murder?

I think it must have been easier in the old days to murder. In the middle ages, you just give someone a bit of a mushroom or something, or you just nudge them over a thing, and there's no forensics.

The only people I'd like to murder are litterers, but it wouldn't be a perfect murder, I'd just get a crossbow and deal with them. Or maybe a nudge, an accidental, "Oh, sorry, didn't see you there," you know? Nudge them over a cliff or something.

In this sort of situation with people getting bumped off, how would you react in real life?

In real life, I think people would be a bit more panicky. I'd be a bit wary, a bit panicky and a bit paranoid and run away and hide somewhere probably. "Do you know what? I'm going home." But it's cute, people just sort of press on, and there's egos involved, and financial incentives involved. And there's a bit of truth in that, how easily people can just go, "Oh, well, they've gone now, let's move on. They would have wanted it. They wouldn't have wanted to spoil the trip."

This is going to come out at around the same time as the Murder On The Orient Express film. How do you think this will compare?

Oh, it will be better. Much better. It'll be funnier... and the production values will be way, way bigger on this one. There will be lots of close-ups of Johnny Depp thinking about things, and it will get a bit boring after about 20 minutes probably.

Who is your favourite detective of all time?

That medieval one played by Derek Jacobi. Cadfael. And the other monk one, The Name Of The Rose. It's just brilliantly written by Umberto Eco, and the ending... it's so literate and clever, and philosophical, and Eco is a great writer. That was one of the best books I've read, actually.