It's because of questions like these that Robert Louis Stevenson's Dr Jekyll and Mr Hyde - in many ways just a common-or-garden Victorian thriller full of stock-in-trade lowlife - still has the power to intrigue me as a writer.
I found myself picking it up again recently. The funny thing is, the original story is very clumsy, particularly the fact that for most of it you don't know that Jekyll and Hyde are one. Hollywood improved immeasurably on this by following Alfred Hitchcock's dictum that suspense comes when you know there's a body in the box all the way through, not when you're told in one moment of shock at the end.
The book's allegory isn't complicated. But somewhere in there there's a grain of prophecy. In a melodramatic way, Stevenson pre-dated Freud with Jekyll and Hyde, making explicit the division between the conscious and unconscious mind.
A few years ago, I wrote my own version of the story - lots of people have done it - about a vulnerable NHS anaesthetist who takes a drug called Euphoria. It was a parable about the Thatcher years. I've also lately become interested in Holmes and Watson, who have a lot in common with Jekyll and Hyde.
Now, looking back, I realise that long before I began using literary sources for my plays, the split personality or alter ego ran through much of my work. Clumsy though it may be, for me, Dr Jekyll and Mr Hyde puts its finger on something very potent indeed.
n Peter Nichols's `Privates on Parade' is currently playing at Greenwich Theatre, Crooms Hill, London SE10. Booking: 0181-858 7755/0181-853 3800.
Interview by Adrian TurpinReuse content