Dumas has total control of the book's narrative – sort of like the Count himself when you think about it. He can switch the focus of the story abruptly, as he does when we're first introduced to Franz d'Epinay. Dumas doesn't do much with the "inner monologue" of his characters – oftentimes when they're thinking something, they simply mutter it.
Having said all this, we should note that at a few points during the novel he lets the characters themselves narrate the action; we get to hear Bertuccio's take on the (unsuccessful) murder of de Villefort and Haydée's account of her father's betrayal. These are exceptions to the rule, though. Dumas is usually reading minds and calling the shots.