Christine worked with the star on Guy Ritchie's action comedy Sherlock Holmes, for which a sequel is already in the pipeline - and out of all the Hollywood A-listers she has worked with she revealed to children's charity Filmclub he is her favourite, although he has high standards.
Christine said: "Robert Downey Jr - he is absolutely the best, and the worst but I certainly don't think in a 'bad' way... he just expects you to be on it - on your game."
"I put that down to what he has been through in his life, and he's disciplined - he's professional, and he will expect you to be as well. It's the same with me, as I barely get to have a glass of wine when I'm doing a film - you're up bright and early; smelling nice and ready to go," she said.
"He's been through it all, and I think that comes through in his acting. He's very committed and very, very professional. He's very versatile."
Oscar and Bafta winner Christine has worked on British films The Boat That Rocked, Happy Go Lucky, Eastern Promises, Casino Royale, The Constant Gardener, Closer, Vera Drake, Secrets And Lies and The Full Monty.
She has also worked on Finding Neverland with Johnny Depp.
She revealed: "Johnny is an absolutely charming man, he really is a ... lovely bloke. He's great and of course very handsome, and very casual with it. It's all about his family with Johnny, and he's very choosy about what he does, he chooses things for the right reason - he enjoys playing oddball characters."