How is the Valjean in Hugo's Les Misérables different from the one in Bille August's film?
The biggest difference is not so much in the essence of the character but in the actions, which are changed in the film in order to incorporate the overall themes within the time allotted. In both the movie and the book, Valjean is a man plagued by early mistakes, mistakes made out of poverty but illegal and immoral all the same. He spends his life trying to atone for those mistakes, but finds himself constantly at war with the law, symbolized by Javert. He is a loving father to Cosette in both texts, and does all he can for her.
In the book, Valjean never confesses to Cosette the truth of her upbringing. He invents a family for her so that she may be married to Marius without any problem. He is terrified to tell her the truth, not wanting to tarnish her opinion of her or to put her happiness in danger. She does find out, but from Marius and not Valjean. Also, Valjean runs off, afraid that the law will come after him still.
In the movie, however, Valjean does confess. He reaches a point when he feels that Javert is about to get him, and confesses to Cosette in despair. This weakness in front of his daughter is the only marked difference between the two stories and the two portrayals of character. On the final point, of Valjean going into hiding, there is a difference as well - in the movie, Valjean witnesses Javert's suicide and behaves as though free forever.