In both the movie and the book, Jean Valjean, in the guise of Monsieur Madeleine, comes upon Fauchelevent, who is trapped under his cart and slowly being crushed by it. In the movie, Valjean, with the aid of a beam he uses as a lever, lifts the cart off of Fauchelevent:
Javert witnesses this feat and afterwards starts to suspect that Monsieur Madeleine is really Jean Valjean, whom he knew as prisoner 24601 and whose great strength he witnessed in an earlier scene where Javert has Valjean retrieve the flag, which was still attached to a mast:
(In addition to this visual parallelism, the two scenes are underscored by the same music.)
In the movie, the main purpose of this scene with Fauchelevent's cart seems to be the stirring of Javert's suspicion. The demonstration of Valjean's compassion and setting up Fauchelevent's later repayment of this kindness are minor in comparison. In the book, however, this scene is a bit different and primarily illustrates Valjean's character.
In the movie, Javert becomes suspicious about Madeleine's true identity only after he lifts the cart, but in the book, he makes it clear that he remembers Valjean's strength beforehand. In Chapter VII ("Despair from the Inside") of Part One, Book Two, the narrator explains that "not one of the galley inmates could hold a candle to him in physical strength. In hard labor, for twisting a cable or turning a windlass, Jean Valjean was equal to four men. He would sometimes lift and carry enormous weights on his back and would occasionally himself replace the tool known as a cric, or jack... His inmate pals had nicknamed him Jean-le-Cric."
In Chapter VI ("Father Fauchelevent") of Part One, Book Five, after Valjean offers increasingly larger sums of money to any man who will get under the cart to lift it off Fauchelevent, Javert remarks: "You'd have to be a monster of a man to lift a cart like that on your back. ... Monsieur Madeleine, I have only ever known one man capable of doing what you're asking here. ... He was a convict. ... From the jail in Toulon. ... I've only ever known one man who could stand in for a jack. And it was that convict."
Despite these comments, Valjean gets under the cart and lifts it off Fauchelevent, basically admitting that he is the man whom Javert remembered.
The other significant difference is that in order to save Fauchelevent, Valjean puts himself in the same perilous position, even though, as the narrator explains, Fauchelevent is "one of the rare enemies Monsieur Madeleine still had" and that he "had done everything he could to hurt Madeleine" in terms of his business. Valjean remains there, trying to lift the cart, even though the crowd in general and Fauchelevent specifically try to persuade him to get out.
The scene in the movie shows Valjean simply doing a good deed that has unintended consequences, but the stakes are much higher for him in the book, where he puts himself at risk by going under the cart and he knowingly stokes Javert's suspicions, all for the sake of a man who is his enemy. Valjean's saving Fauchelevent under these conditions illustrates his extreme selflessness.
