All I can recommend is to read/speak your dialogue aloud. Not whispering, not muttering, OUT LOUD. (Virginia Woolf used to try out her dialogue in the bathtub, which greatly entertained the cook downstairs.) This will help show you what’s fakey, hokey, bookish — it just won’t read right out loud. Fix it till it does. Speaking it may help you to vary the speech mannerisms to suit the character. And probably will cause you to cut a lot. Good! Many contemporary novels are so dialogue-heavy they seem all quotation marks — disembodied voices yaddering on in a void.
URSULA K. LE GUIN