Boris has French ancestry, and presumably received reasonable instruction in French at whatever second-rate school he went to. Note, though, that in this interview Boris switches into English when it all gets a little too difficult.
Jodie Foster was educated in French at Le Lycée Français de Los Angeles, as well as spending a lot of time in France as a child. Hence her French is pretty much perfect. She dubs most of her films into French herself, and even recorded a few pop songs in French when she was younger.
Churchill's radio address of 21 October 1940 to the beleaguered French people is known as Dieu protège la France. Churchill had a lot experience with French, having a fluent mother and wife, studying French at school and using French regularly in diplomatic circumstances.
Nevertheless, it's clear from this recording that Churchill made no attempt to use a French accent. Indeed, he is said to have gone out of his way to pronounce French place names in as English a manner as possible. De Gaulle once said that it was by learning English that he was able to understand Churchill's French. Incidentally, that's often how I feel some of my language exchange partners are able to understand me!
Audrey Hepburn was born in Belgium and spent part of her childhood there. She was a polyglot and in this video she speaks English, Dutch, French, Spanish and Italian.
Canadian actress Evangeline Lilly is not from Québec and does not speak French with a Québec accent. Instead, she learnt French through immersion programmes from a young age.
Malkovich lived in France for a decade, but left in 2004 after a dispute with the French government over income tax. He uses the same signature somnolent drawl in French as he does in English.
I've talked about Bradley Cooper's French on the blog before. He studied French at university and spent six months in Aix-en-Provence as an exchange student. In this video he talks about The Hangover Part II, called Le Very Bad Trip II in French!