Sandra Bullock is one of a handful of Hollywood stars joining Despicable Me's lovable, pill-shaped creatures the minions - as they take centre stage with their own movie spin-off.

It's been 17 years since Sandra Bullock last starred in an animated film, lending her voice to Moses' sister Miriam in Dreamwork's Prince of Egypt. But when asked about her decision to join the cast of Minions, she couldn't have responded quicker. "My son," she smiles. "I wanted to do something that he could see, that I was in."

Bullock has an adopted son called Louis, who travelled with her to London ahead of the movie's world premiere. It sounds like a glamorous life for the much-papped five-year-old, but Bullock points out that "he doesn't know what I do."I've started telling him that I go and make stories, but he doesn't quite care what that's about - nor do I want him to."

She gushes that her son loves physical humour and animation, and because she loves to hear him laugh, joining Minions was a no-brainer.

The film is a spin-off prequel to Despicable Me and Despicable Me 2. The latter alone took $970.8 million (£616m) at the worldwide box office, and it was the top grossing film of 2013 in the UK. "I'm so used to using a prop or a person or my body to sell something and I couldn't sell the character that way. I had to only use my voice, which was tricky."