Want to learn to create awesome dialogue that will pull readers in and differentiate you from the pack of writers whose characters all sound the same? Here are some tips!
Think about what each of your character's attitudes are: Are they happy? Are they sad? What I mean is are they are a happy or sad person overall? Do they take a gloomy perspective to life or an optimistic one? Do they find barbs in every compliment? Or are they the sort of person who pulls others up out of bad moods?
Think of who your character is and how they approach life. Then fool around with writing some dialogue by creating a situation that this kind of character would be faced with.
For example: a character who thinks the whole world is against him has to ask for help when his car breaks down.
A character who is perenially happy is going to approach this situation differently, as is a person who finds sexual innuendo in everything -- or a person who very much has a victim mentality. What about the person who gets off on gossip. Or that person who always must one-up everyone on everything. You get where I'm going. Think about the attitude with which your character approaches life because this is going to inform the kind of dialogue he or she uses.
Related to this is what your character's background is. Obviously if you are a professor of English you are going to have a different way of speaking than a guy who fixes cars for a living. (Although a great way of creating a deep characters is making your mechanic speak as if he were a college professor.)
Sure, we don't want to stereotype too much. Still, how much education we've had, our social class, etc., does often have an impact on how we speak. And these differences can be subtle. A professor of English at the local community college might speak differently then one from Harvard. The same professor who comes from the South may speak differently than someone from the North.
Again you can brainstorm the different ways that different characters might ask for the same thing. Say, your characters all want a cup of coffee.
The elitist wants a cup of French-pressed Ethiopian roast with no cream.
The coach dad wants a cup of joe.
The New York taxi-cab driver wants a cup of caw-fee.
The Brit wants a cuppa.
You get it. And then does your character say thank you when they get their coffee? Or do they just take it and drink it without saying anything? Or do they even just send it back because they don't like it? All this is what your character can be saying that lets the reader (or audience if this is a screenplay) learn about who your character is without having to use a lot of expository sentences to get across the same idea.
Another great way to distinguish characters is by creating a catch phrase that you character always uses. I learned this in a character improv class, but you can also apply this to your writing. Is there that one thing that the character always (or often says) that can distinguish him from everyone else. This is especially true when writing screenplays. You want a reader to be able to cover up the character names and STILL be able to get an idea of who's talking.