This question is asked fairly frequently around here, and if you do some searching, I'm sure you can find the threads. The answer I always give is that you need to spend time with them, which means writing scenes with them. It doesn't matter whether the scenes ultimately show up in your novel -- if your novel takes place during, for example, one year of your character's life, when he is 35, you could write a scene from when he is 15 or 20 or 25 that involves something important in his life -- the day his sister dies, or the day he meets his boyfriend, or the day he graduates from med school, or whatever it is. Maybe you don't ever mention that he has a dead sister in your story, but you know it, and it can inform his experience and how he feels about things. Maybe it's only alluded to or briefly mentioned in the book. Or maybe he's the guardian of his nephews, so that was a significant occurrence. You can discover all kinds of things about your character and potential storylines this way.
Exactly as Liz said. The best way to understand a character is to write about them. If you know about an event that happened some time in the character's past, particularly one that's going to effect the story, write them out. Get to know their personality in the same way that you're going to portray that personality to the world. I always keep a short list of different ideas for things I can write about for my characters. Sometimes they get done, sometimes they don't. It's just something to get my brain churning. If it's an adult character, it might be something like "Their First Job" or "How They Met Their Significant Other." Things that aren't going to be necessary but that, if all else fails, might inspire me to get something down about the character that helps me to learn more about them. But that's just me. ^.^