Dialogue is notoriously difficult. You have to make each character distinctive. The speech needs to sound natural. Unlike you, characters can’t deliberate about word choice. Here are some tips that may help.
DON’T have characters speak grammatically correctly. Dialogue reveals much about your characters, and when they talk like scholars giving lectures, their speech sounds contrived. Even college professors don’t got to follow no rules when chatting with friends. Vary grammatical mistakes for distinctive voices. One character can’t use the right prepositions for save his life. Another get subject-verb agreement wrong. Even if characters speak proper English . . .
DO have them speak in sentence fragments. Ordinarily we take pride in well-crafted sentences, and perhaps a character, say an academic, needs to be erudite or pompous.