I'm gonna be a little contrarian. I don't think dialogue has to sound "real" to be real and true to the characters and the world in which they live in. I think about Shakespeare or even Mamet. Obviously, no one talks like Shakespeare--they didn't at the time, like, who the fuck speaks in iambic pentameter? And very few people speak like Mamet rights. But the characters they created are real and whole, with clear motivations and desires, and are actively pursuing those. But, to be an agreeable sort, @Maurice and Tony Gilroy are right, it's about knowing people, it's about being an observer of real people, but, they can still be in unreal situations--I'm a made up creature walking to Mount Doom singing silly songs to get rid of a ring--but still feel real and whole. I feel like, if there's an emotional core with a full world then dialogue can sound "unreal" and still be real. If that makes sense.