1. Look to the Script.
Okay, about 85% percent of you are going, "Duh Mike, of course you should read the script to learn about your character." You are correct, but I put this up here anyway because some times you have to dig a bit deeper than what is on the surface. For example, not that long ago I went almost an entire rehearsal process without realizing that my character's parents had divorced and his dad had remarried. How on Earth did I miss something as big as that. The information was in a single line that is never mentioned again and didn't specifically state that his dad remarried and I was busy playing the line for a laugh. Now come show time I still played that particular line for a laugh, but the knowledge I gained from it informed many of my other choices. Now I am sure some of you are asking, "But Mike, what if my character doesn't have lines, or the script doesn't give me anything to work with?" Well that leads me to the second option:
2. Create your own backstory.
All right, so you are Spear Holder #7, what makes Spear Holder #7 different than Spear Holder #6? Odds are the script doesn't tell you. Well here is the good news, that means you get to make it up! Does Spear Holder #7 have a wife and kids at home, maybe he simply wants to get through work without getting injured (objective, remember those?) so he can see them again. Or maybe he grew up in the military and is a true believer in what he is protecting and always the first into a fight. Even if you never actually get into a fight during the show these choices will still inform your character. A great game you can play with other members of your cast is Character Questions. Everyone sits in a circle and asks each other questions, and everyone has to answer as their character. this can force you to make choices. But remember that until show time you are not locked into your choices, rehearsal is there to try different things, don't just settle for the first thing you come up with.
Suppose you really can't come up with anything. One trick you can use to give your self a starting point is choosing an Archetype for your character. Archetypes are like Stock Characters in the theatre. The tough guy, the bully, the good girl, the bad girl, the bad boy, the hero, the dreamer, etc. The stuff you see in almost every movie, tv show, play, and read about in almost every book. Choose one of these archetypes for your character and see how the play feels doing that. Now understand archetypes are very general and therefor do not lead to the best performances when used on their own. You want to use an archetype as a way to start thinking about the character and then get more specific from there and if you leave the archetype behind entirely that is fine, it is just getting your creative juices flowing.
So there you go, just a couple ideas to help with character development. Until next time!