Chris' problem is that ginger-headed dorks don't command much in the way of instant respect from people like that, let alone fear. Yeah, well, think again, annoying people: your days are numbered.

Ben Wheatley can see why his macabre comedy Sightseers has been compared to Natural Born Killers: both films are about killer couples who take to the road. It is impossible, however, to imagine Woody Harrelson's Mickey choosing Chris' holiday itinerary, which takes in a tram museum, a cave complex and a museum in Keswick that explores the history of the humble pencil. Nor can you picture Juliette Lewis knitting herself some crotchless underwear, which is Tina's idea of eroticism. Tina hasn't picked Chris as a serial killer, her experience in that area being limited. As it turns out, however, she may have more murder in her heart than he has.

This is Wheatley's third film about covert killers: Down Terrace, his first film, centred on a crime family, while the characters in Kill List were a paramilitary cult. ''These are groups of characters who define their own laws,'' he says. ''They exist within society but don't really care about the rules of society.''

Chris and Tina were not his inventions - the actors Steve Oram and Alice Lowe, who play them in the film, developed the characters for a stage sketch show - but they certainly bear his stamp. Quite ordinary people, he believes, are capable of almost anything.

''You know, evil isn't a man in a black cape with pointy teeth, it's everywhere and everything and informed by good, as well,'' he says. ''I think it's that thing that you can be a rational, intelligent person and then do these things that are so incredibly stupid that you can't quite believe you've done them. And I think everyone's capable of that. He's quite an intelligent guy, Chris, but he does some stupid things. And some bad things, but he's also a good person.''

Desperation, in particular, pushes us in unexpected directions. Chris, we discover, was retrenched before he met Tina. It is true that he sets up strangers as his imaginary foes.

''But when you're in that problem, when you have that, you seek people to blame for it - and it's easy to blame people who are more privileged than you.''

Wheatley has never so much as been in a fight, but he looks back with some perplexity on the moment a few years ago when it seemed the banks were about to fail. Without access to money, he reasoned wildly, how would they buy food? How long would the food in their cupboards last? Two days? Three? He bought a couple of catapults. ''Ha! Luckily, I didn't need to use them. But that kind of stuff is certainly in my films.''

There is a distinctly British chip-fat whiff of the films of Mike Leigh here, although Wheatley's own references include Alan Clarke's films of the '60s, such as Rita, Sue and Bob Too: social comedies tinged with despair. What they don't have is the hilarity of gore; Sightseers has as much in the way of spilled brains as the slender budget allowed. Comedy and horror have much in common, Wheatley agrees: both genres work on ''gags, set-ups, payoff and timing''. And both are about pain. ''Most comedy is about someone else's misfortune.''