When you have a character talking, have two things you know about their lives in your head as you let them talk. Two things that make them what they are. What was their childhood like? What was their first job? Do they spend a lot of time alone? Are they guarded around people? Because dialogue is about moving information around and expressing character. What you know about them affects the way they talk. Take a book you like — or, hell, even one you don’t — and select a passage of dialogue, and see what you can learn about those characters from the way they speak. (And, on top of that, see if the way they speak changes during the course of the book.)
Via Warren Ellis.