In A Monster Calls rising star Lewis MacDougall plays a 12-year-old boy grappling with his mother's fatal illness. It's an unusual role for Weaver, who says she was drawn to the film for a variety of reasons, including director J.A. Bayona's previous films The Orphanage and The Impossible.

Sigourney Weaver in A Monster CallsSigourney Weaver in A Monster Calls

"Frankly, I end up doing a lot of big movies," she says, "and I so wanted to do something very small and intimate and very dramatic. The story really catches at your heart. It's not a film about cancer but about coming to terms with loss. I just knew Bayona would create something very original and so personal. And I was drawn to the character."

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When she enters the story, her character is rather pushy. "She's an old-style grandma," Weaver laughs. "She is very strict and not very empathetic in the beginning. She doesn't have a very good relationship with her grandson. One of the reasons I wanted to be part of it is that very rarely do you have a chance to play someone like that who then goes on a journey. You see what's underneath all that armour. Often times, especially playing an older character, you would just see the unsympathetic part, but this movie takes care of all of its people!"

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She also enjoyed the way the story approaches a heavy topic. "We make a big mistake if we sanitise this stuff," Weaver says. "Children can tell what's not real and also, I think when you protect them they get more afraid because then they go, 'What's happening? There are monsters!' So I think in the course of this movie you see the power of the imagination and how this little boy needs to really come to terms with the complexity of this experience for himself."

Weaver doesn't see any difference between a smaller film like this and her bigger franchise roles in the Alien and forthcoming Avatar blockbusters. "I think genres are very misleading," she says. "I think when you start off from the idea of a movie being part of a franchise, you're going about it the wrong way. First you need to have a really great writer. You have to start with one really good script and really good characters. Look, franchises have to be earned. Otherwise, it's going to be put together by Hollywood people, adding a little this and that. I just don't think that works."

Watch the trailer for A Monster Calls: