Maiming a horse is a far cry from riding a hippogriff, isn't it?

Dan Radcliffe: Completely. There came a point on the third Potter film when I thought, acting is what I want to do. I thought it would be a bad idea to wait till the Potter films were all finished to do something else. There are certain people who will be more than happy to see you in any other role you like, and there will be some who will never, ever see you as anything other than Harry Potter. Once you've accepted that, it's fine - you just do whatever you like. With this, they can say I'm good or terrible but the one thing they can't say is I haven't challenged myself.

And playing a horse is a long way from one of Tchaikovsky's swans...

Will Kemp: What was intriguing was coming into such a perfect play where I'm able to be an actor and still utilise the training I've had as a dancer in physical theatre. So I get to cross over. But I've also been working with a lead choreographer in a way that I haven't previously. She does a lot of contact improvising, which climaxes with people throwing themselves at each other.

For the older cast members, are there strong memories of the original production, and does the play still have a contemporary urgency?

Richard Griffiths: I was at the RSC in 1973 knocking out 12 or 15 shows a year at Stratford. A night off when you could go and see somebody else's show was just a futile dream. So I never saw Equus. Based on what people told me about it, I wondered whether the psychiatry world has moved on a bit. I have friends who are psychiatrists, and I ran it by them. They said there are new concepts and new problems, but essentially the approaches used are all perfectly valid and would be recognised by any practising psychiatrist. For Peter Shaffer (playwright), the key question is nothing to do with psychiatry. It is much more fundamental and philosophical. It is to say, what is the purpose of us being here?

The play argues that if you scratch the surface of any domestic environment, you will find abnormality and dysfunction. Is that your experience?

Dan Radcliffe: I know a few people that are very much like Alan, in that they are quick to anger. I recognise bits of myself in Alan, absolutely, and I think when young people come to see the play they will see themselves in him. I've got a great image of somebody who came to see the play when they were 16 and saw themselves as Alan (Radcliffe's role), and then coming to see it again and seeing themselves as Dysart (Griffiths' role).

Richard Griffiths: It's nip and tuck as to who is in more trouble. Dysart really wants to take a walk on the wild side but he knows it could be the end of the world if he did.

Dan Radcliffe: Alan's trouble is more visible because he hasn't got the vocabulary or social skills to disguise it.

Richard Griffiths: We are all products of various degrees of dysfunction. There is a big debate in the play about what is normal. My guideline is Billy Connolly's: "I'll tell you what's normal. There's you, and there's the rest of the f***in' world. That's what is normal."

The play is an extreme portrayal of teenage isolation. How do young actors avoid the psychological fall-out of early stardom?

Jenny Agutter: For me it was very different from what Daniel has done. I started at 11 and was 17 when I did The Railway Children. I was very questioning as to whether I should do it because I'd just done Walkabout, which is about the loss of innocence, and I was going to move on. But you go between being young and older at that age, and it seemed the right thing to do. Daniel must be one of the few people that's actually allowed to grow up in a character.

Dan Radcliffe: It's me and the boy who was in The 400 Blows and Truffaut's other Antoine Doinel films. I've been lucky enough to have a direct route laid out for me.

Jenny Agutter: You are probably much more clear-sighted with what you are doing with your acting. I was really quite tentative about it.

Physically, it's a hugely demanding first stage role for a 17-year-old actor.

Will Kemp: Dan is up for anything. We do a lot of things together - running around with him on my shoulders. I'm grateful that he's in fantastic nick, plus he's actually quite light.

Dan Radcliffe: Eight and a half stone (119 pounds).

Will Kemp: I've probably carried heavier women than Dan.

Towards the end of the play, Alan and a stable girl have a love scene that goes badly wrong. The scene is, in all senses of the word, revealing, in that it involves nudity. How do you approach it?

Jenny Agutter: This is always the difficulty with people's obsession with the idea of nakedness. They forget what it's actually about. In a way, it has to be naked. It's not sensational. When this play was on, there was Abelard and Eloise, Hair, Oh! Calcutta! They were all taking their clothes off.

Dan Radcliffe: I didn't look at the nudity and go, oh great. But it's the same as doing a role with an accent or a particular affectation. You look at the character first. Lots of the actors that I've admired have at one stage or another taken their kit off. It's a rite of passage. That iconic scene is the physical and emotional climax of the play. So if I do that with pants on, it would be crap.