Ever picked up a book and halfway through it realized that all of the characters sound remarkably the same?
I know I have.
As an author, that's an easy mistake to make. After all, every single character is coming from you!
Here's a simple trick to make sure each of your characters have their own unique diction/responses/reactions/expressions:
Go through a few dialogue-heavy scenes in your novel and highlight each character's dialogue with a different color (you can do this right on the computer or print it off if that's easier). Then read each character's dialogue on its own. If you've done it right, the cadence and rhythm for each will be different and you won't have them sharing similar expressions or reactions to the external conflict.
If you notice any of the separate highlighted strands sound alike, go back to your characters, figure out what makes them fundamentally different from each other, and change their dialogue to reflect that.