Ben Stiller doesn't think he has a ''drive'' for acting any more.

The actor-and-director enjoyed making his latest film 'The Secret Life of Walter Mitty' - in which he plays the titular lead character - and he wonders if his passion behind the camera has taken over.

He said: ''I don't want to ever be dismissive of it. To be a really great actor takes as much commitment as anything. But I do wonder sometimes, for me, if I have as much drive for that these days as I do to direct, because that I don't even question. But then it's also about finding the projects, and it's much more of a commitment.

''But you're not limited by being cast in something, what type you are, what roles you can play, because you are who you are, which is fine, but directing to me is so many possibilities, so many worlds to explore.''

The 48-year-old star admits he's becoming ''obsessed'' with directing because it forces him to focus on something.

Speaking to the Sunday Times newspaper, he said: ''Directing is about obsession. Where it can start to hinder you is when you obsess on something you shouldn't be obsessing on. Or obsessing on something because you're avoiding something else. It's all about that anyway. It's like that line from 'Manhattan': We're all just keeping busy because we're all going to die and we don't want to think about it.''