Reese Witherspoon feared for her co-star Mark Ruffalo's life after he fell out of a second-storey window on the set of forthcoming movie Just Like Heaven.

The Legally Blonde beauty, 29, was tricked into thinking COLLATERAL actor Ruffalo had plunged to his death from the tall building - and was relieved to discover he was pulling a prank.

Witherspoon says, "We were doing a scene and Mark said, 'I can't stand you, I want you to get out.' Then he jumped out of the window.

"We were on a second-storey platform up in the air, and he's got the stuntman to put out the mattresses and not tell us.

"We all screamed, then he popped up and jumped back in the window.

"It was so funny. He was always cracking up and doing silly things."