If I read the book first, I have a picture of the character of my own devising, which may or may not get overridden by the film's character Usually it will unless I disagree strongly with the film's interpretation.
If I watch the film first then when I read the book I can't help but think of the film character.
i read da vinci code and seen someone who could have been tom hanks but for me, hhowever I imagined a totally different female character for the leading lady. I sometimes cant watch a film where a character is completely different to how I imagined. Especially if its a book that I have been completely absorbed in. [stupid emoticon]