I think lead, she is the main carachter of the film. I liked the trailer a lot, cinematography is great and seems there are few differences from the book (it is like the book really). Affleck is count Pierre and not the King that is played by a young actor, also the atmosphere is very similar. I hope we'll see some point of views and not just Marguerite's, because it's the people who have to decide the side.
In one shot we can see Marguerite pregnant, too.