Bradley Cooper can't stop mimicking Robert DeNiro.

The actor appeared alongside the screen legend in new movie 'Limitless' and admits he couldn't help but copy his co-star's mannerisms - even when they had finished working on the film and he had started shooting 'The Hangover 2'.

Bradley said: "It's like osmosis. You can't be around that guy without starting to take on his mannerisms. And I started to worry that during the scenes that I had with him I'd start doing him - to him - in the scenes. Not even consciously.

"So much so that when we started filming 'Hangover' part two - like when we're in Bangkok and I'm talking to Alan, and he's like 'Cut! Dude, you're doing De Niro.' I'm like, 'I know, it's crazy.' "

The 36-year-old star also promised the forthcoming 'Hangover' sequel will be "epic".

Asked what fans can expect, he said: "You know, I think it might be epic. I think it might be a big one. I think.I shouldn't say - I've got to stop saying that."