George Clooney thinks it's hard to define what makes a movie star

Amal Clooney and George Clooney have been married since 2014. Photo / Getty Images

George Clooney thinks his friends Brad Pitt and Julia Roberts are "proper" movie stars but he finds it hard to articulate what makes an actor fall into that category.

Asked what makes a movie star, he said: "I don't know. I can't comment on that because I can't comment on how people perceive or how I am perceived by people.

"I can comment on movie stars in general. I know a few of them and I've known some big ones. I was really good friends with Gregory Peck, he was a big one. Newman, there's some other ones I'm friends with who are movie stars, proper movie stars.

"Brad Pitt's a movie star, Julia Roberts is a movie star.

"I suppose the only thing—I can't remember who it was, a government official once said, you know I can't give you the definition of porn but I know it when I see it.

"It's sort of that way with those stars, there's not one specific thing, there's something intangible about them that I see, that is impossible to describe it's just… You watch Gary Cooper who is an interesting actor and Bogart and actors like that and you go 'why are those guys movie stars?

"He doesn't look like Cary Grant,' but you couldn't take your eyes off him.

"Same thing with Spencer Tracey, you couldn't take your eye off that guy….

"Sandy Bullock's a movie star, and I worked with her and I've known her since she wasn't a movie star when we were both young actors and even then there was something about Sandy that you just knew she was going to be a hit."

The 59-year-old star is grateful he's never had success in a major movie franchise because it's stopped him from being typecast.

In an interview for BAFTA's Life in Pictures, he said: "I was lucky in a weird way that as an actor I never got massively successful in anything, you know, in a funny way.

"I never was - I did an action film like 'Peacemaker' and it wasn't a hit, if it had been 'Die Hard', which it wasn't, then that's who I would have been.

"I would have been the action guy.

" I did 'One Fine Day', if I'd done romantic comedies and any were a massive hit, I would have been the romantic comedy guy, and then I couldn't have done drama and the other way I couldn't have done comedy.

"Because they weren't, and if you go through my career a lot of the things weren't home runs at all, it's allowed me to do and try other things.

"I'm allowed to do a comedy or a drama, so I can do something as whacky as 'O Brother Where Art Thou' and something as straight as 'Michael Clayton'."