I have learned to use the way characters speak as a distinguishing mark, especially L'Evallier who will not use contractions at all. Even the Fairy Queen will sometimes, especially if she is stressed, but never L'Evallier. It adds to the poshness of his character too. Dialogue must be appropriate to the character. It is apt for Eileen to swear like the proverbial trooper if she's having a bad week with her cousin, the Queen, or with fighting dragons or both (!) but it would not be for L'Evallier. He would become silent. Literally the strong silent type.
So how well do you know your characters then? Do you know how they would speak? What kind of language they would use? Do they adopt one type of speaking for public use and another for private? What makes them drop their guard and say something unexpected? These can be useful prompts. Hope you have fun using them.