As with the Jack Nicholson film version, there's some steamy coupling required from the principals. How does he feel about having to shake his tush every night in front of 500 people?
'Eight-hundred people,' he corrects. 'And five. And probably some of the crew will sneak around, so we're up to about 812. No, I don't try to shy away from what's difficult in a character. Charlotte [Emmerson, his co-star] is the same. I like that stuff.
I like forcing myself to learn and be challenging to my partners, too.'
Hence, perhaps, his rumoured awkwardness - a reputation of which Kilmer is wearily aware.
'It gets written that there's some sort of reason not to hire me,' he says. 'I haven't heard any complaints from good directors.'
Could there be times, though, when his intensity boils over into selfishness? 'I've never had a selfish intent acting,' he insists. 'I like actors. I really love making stories up. But I'm more aware nowadays of how lucky I am. I'm sure I was more selfish when I was younger - about everything.
'But I feel the only thing that is legitimate criticism is that I was challenging to directors in a way that made them feel threatened. It's a drag to have to talk about it because I'd rather talk about Michael Mann or Al Pacino or Robert De Niro or Brando - people whom I've worked with who are really great. It seems I have to talk about idiots all the time.'
The West End, he hopes, will be a chance to draw away from the fog of misconception. 'Theatre is so pure,' he says. 'I came here to see theatre when I was 14. And to be able to come back and tell a story about where I'm from is a real privilege.'
The Postman Always Rings Twice, now previewing, opens Jun 7, Playhouse Theatre, Northumberland Avenue WC2, Mon to Sat 7.30pm, Wed and Sat mats 3pm, £8 to £40. Tel: 0870 060 6631. Tube: Embankment