Helena Bonham Carter has compared Sacha Baron Cohen to a "hyperactive child".

Speaking to the Sydney Morning Herald, she said that Sacha, 44, is "totally unpredictable, like a hyperactive child sometimes [but]I'm not saying that, I'm not".

She added: "He is very inventive, and he's fun, but ... I'm pretty direct with him. Sometimes I think what Sacha doesn't appreciate is that he's just telling a story. The weight isn't always on his shoulders. A lot of the time it's just playing the scene, not inventing the scene. He comes from such an improvisational background. He can't but help improvise. It's interesting."

Helena, 50, plays the Red Queen in the new movie, while Sacha portrays Time and she admitted filming on a huge soundstage against a green screen for CGI made things difficult.

She explained: "There was not much atmosphere. You know the problem is this room is huge, and you see everything. It's generalised lighting, so there's no sense of focus, there's no sense of, 'OK we're imagining'.

"On most sets you realise where the drama is. What's in front of, what's the imagined reality and what's off, but here there was no demarcation between me and Sacha trying to work in, and seeing absolutely every other member of the crew basically bored out of their head, I don't blame them for playing Sudoku. It was hard because it was completely sterile."