The most important thing about writing for theater is to realize that a theatre script is a half product, not a finished one. And the second is that the text will be spoken out loud. Other people will come and take it and have their way with it. They will interpret it, pronounce it, they are probably doing something while saying it. And there will be an audience watching and listening. A very different experience than reading a book at home on a quiet Sunday morning.
The writer will have to be able to let go. Sit back and relax and hopefully be surprised and delighted by what actors or directors do with their text. Sometimes you’ll find that there is much more dept in it then you first expected and other times you will want to rewrite after hearing it out loud, because that beautiful sentence on paper just doesn’t sound natural when spoken out loud.
Real people don’t speak in full and grammatically correct sentences. Often, they say the same word many times, they stop midsentence, get distracted, change subject, rephrase.
Also, people don’t say what they really mean. They might talk about the weather or an object or food when trying to explain something else. The same person will speak in a completely different way while trying to get a pay raise from their boss then when he or she is having a discussion with their spouse about whose parents they will visit for Christmas.
You create your characters by the way they speak and what they say, not by describing them like you would in prose.
Start by listening to people's conversations or even secretly record them and listen back. You can do this during your lunchbreak, while waiting for the bus, at a party, anywhere. If you have never done this before you will be surprised how weird people actually speak and how much it tells about who they are.