Actor Daniel Radcliffe gave fans a shock by making a surprise appearance at a screening of his new film What If.

The Harry Potter star makes his romantic comedy debut in the new movie opposite Zoe Kazan, and he showed up at a recent preview of the film at a cinema in Los Angeles to see fans' reaction first hand.

Small groups of fans were selected to do a post-movie interview with Buzzfeed.com, and during the chat, Radcliffe, who called the prank a "cunning and hopefully joyful act of deception", stealthily crept up behind them.

One fan initially ran away upon the sight of her favourite actor, while another confessed her first instinct is to "punch people in the face" if they sneak up behind her.

The actor enjoyed surprising the fans, but admitted he feared the viewers would confess they hated the film right before he revealed himself.

Radcliffe explained, "So that was me scaring the s**t out of a bunch of people, which is always fun. It is nice to get that kind of genuine reaction from people. I was definitely sitting back there worried that they would be talking about how they hate the film and they wouldn't be at all pleased to see me."