When the Harry Potter star played mentally disturbed Alan Strang in Equus on London's West End last year (07) he discovered he suffered from a phenomenon he calls Michaelangelo's David Effect - named after the the biblical battle between David and Goliath.

Radcliffe explains, "He (David) wasn't very well endowed, because he was fighting Goliath. There was very much that effect. You tighten up like a hamster.

Rather than employ any trickery to make himself appear bigger - Radcliffe has learned to accept the shrinking as a necessary side effect of being naked onstage.

He says, "The first time it happened, I turned around and went, 'You know there's a thousand people here, and I don't think even one of them would expect you to look your best in this situation."