Stop writing. Put down your pen, or take your hands off the keyboard. Close your eyes. See each of the characters that are talking in your brain. Don't try to imagine them doing anything. Stop holding them back. Listen to what they are saying without judgment. Then write down and record their words just the way you hear them. If you put words in their mouth, it's not a true statement of what they would say. Just get out of the way of your characters and let them talk. They'll tell you what you need to know.
If this process doesn't work for you, then start listening to more conversations. I don't mean hearing the ideas being expressed, I mean pay attention to the bits and parts making up the dialogue. You could look at a book and think, "Oh, that book is about history;" OR you could look at a book and say, "That book is bound in cloth and has gilt lettering on the cover." Listen to conversations the same way as you can see the book as an object all by itself, to learn how the nuts and bolts of conversation fit together. What would these people, in this time frame, in this area of the world, sound like if they were talking to each other? There you go.