Why, though? Not only does she have more screentime than Dunst, she also has the entire second half of the film devoted to her perspective. It's a film reliant on both Claire and Justine's experiences and how they parallel.
Feel free to jump in, other people who have her supporting.
I mean I can't defend that placement. I'm hazy on the film and didn't care much for it, but for whatever reason I always had Gainsbourg in supporting. Don't know why, that was just my impression at the time. Dunst just felt like the central character to me, but admittedly I might have been a bit checked out by the second half. I remember a lot more about the first half.
Berenice Bejo, The Artist - SUPPORTING Jodie Foster, Carnage - LEAD John C. Reilly, Carnage - SUPPORTING Christopher Waltz, Carnage - SUPPORTING Kate Winslet, Carnage - LEAD If you want to vote the cast of Contagion be my guest, don't have time to list everybody - EVERYONE IS SUPPORTING Gerald Butler, Coriolanus - N/S Ryan Gosling, Crazy, Stupid, Love - SUPPORTING Viola Davis, The Help - Really difficult but I'm gonna go with SUPPORTING Cecile de France, The Kid with a Bike - N/S Kevin Spacey, Margin Call - N/S Charlotte Gainsbourg, Melancholia - SUPPORTING Andy Serkis, Rise of the Planet of the Apes - SUPPORTING Elena Anaya, The Skin I Live In - SUPPORTING Jessica Chastain, Take Shelter - SUPPORTING Brad Pitt, Tree of Life - SUPPORTING Chris New, Weekend - N/S Alex Shaffer, Win Win - N/S Tyrannosaur: Original or adapted - ADAPTED
LEAD: Berenice Bejo, The Artist // 5-3 Jodie Foster, Carnage // 6-2 Kate Winslet, Carnage // 6-2 Viola Davis, The Help // 5-3 Charlotte Gainsbourg, Melancholia // 5-2 Chris New, Weekend // 3-2-1(crap)
TIE: Andy Serkis, Rise of the Planet of the Apes // 3-3 Matt Damon, Contagion // 2-2
SUPPORTING: John C Reilly, Carnage // 3-5 Christoph Waltz, Carnage // 2-6 Jennifer Ehle, Contagion // 1-3 Rest of Contagion cast // 0-4 Gerald Butler, Coriolanus // 0-3 Ryan Gosling, Crazy, Stupid, Love // 1-5 Cecile de France, The Kid with a Bike // 0-3 Kevin Spacey, Margin Call // 1-2 Elena Anaya, The Skin I Live In // 0-5 Jessica Chastain, Take Shelter // 0-8 Brad Pitt, Tree of Life // 0-7 Alex Shaffer, Win Win // 0-4