Daniel Radcliffe and Richard Griffiths, stars of the Harry Potter movies, are to appear together on the London stage in April, in a new production of Equus, one of the defining plays of the past half-century.

In one scene, Daniel will have to perform a sexual act while naked and mounted on a horse - but by all accounts, he’s not daunted by the prospect.

Teenage Radcliffe has become a household name the world over since winning the part of J. K. Rowling’s schoolboy wizard (he has played him in four films and is in the middle of shooting a fifth, The Order Of The Phoenix).

As I exclusively revealed in December, he will play Alan Strang, a troubled youth institutionalised for blinding six horses with a hoof spike.

This seemingly senseless act is at the heart of Peter Shaffer’s extraordinary 33-year-old drama, which explores the nature of passion, religion, equine worship and teenage instability.

It was a sensation when it first ran at the National, then on Broadway, and later in a movie starring Richard Burton and Peter Firth, who created the part of Strang on stage...