- what their voices sound like
- how they respond to others
- what kinds of phrases they use
- whether or not they use proper grammar
- what things they say are funny, or interesting
- how their manner of speaking changes depending on who they're around
By observing the way people around you speak, you can write better dialogue. Although I love Young Adult fiction, many books in this genre are guilty of having very bad dialogue. Like, really, really bad.
Nothing will distract a reader from your story more quickly than conversation that sounds stilted and weird. Not every person speaks in the same way, and neither should your characters. Sometimes writers forget to think about the way a character speaks and when this happens, that character usually ends up sounding like a weak version of the author themselves. Not really what you want. Making good characters is like making a collage. They will always have parts of you in them. If you're writing truthfully, they kind of have to, but memorable characters are not solely reflections of the author. They are bits and pieces of ideas and phrases and opinions and people coalesced into one juicy mess of a person. A made up person, but a person nonetheless.
Read your dialogue out loud and ask yourself if it sounds too much like you. It should unmistakably sound like something your character should say, even the "OKs" and the "buts." If you find that you don't get a clear picture of a character from the dialogue in your writing alone, it's time to do some research. Go watch people. Maybe your character never uses contractions. Maybe your antagonist speaks slowly and in a measured voice, like Alan Rickman. Maybe they speak quickly, like Sherlock on the BBC series. If you don't feel like going out, reread a book you love and pay close attention to the dialogue, or curl up with your cat and watch Sherlock on Netflix--that show is the monarch of witty banter. While I wouldn't advise peeking through keyholes, sometimes eavesdropping can be good for you.