One technique you can consider using is to interview your characters. Get to know them for who they are. By doing so, you can begin to understand why they do the things they do in your story. Remember, you really don't need a huge backstory to understand your characters. Their reasons for doing things can be pretty basic.
Now, when you are conducting these interviews, go beyond simply yes and no answers, or ones that allow the characters a chance to answer with single words.
- What did you want to do when you grew up?
- Describe your favorite holiday? What is it that makes that single day so special?
- Tell me about a day you and your childhood friend did something completely stupid?
- What is one secret you have from your childhood?
Once you know this information, do you need to include it in your story? Depends on if you need to or not. More than likely, you won't need to add full stories into the main plot of your book. Doing so will really detract from the purpose of the book. But, dropping little nuggets in here and there will give us a sense that you know who this person is.
For example, they walk into a room with a buffet table set up with food. In all honesty, will they love everything on the table? Probably not!. Will those senses spark a small memory or trigger an emotion for the characters? Probably. Will that lead to an action or a comment in the story? If it is necessary to the plot, then go for it.
Just learn to see your characters in 3 dimensions and you will be amazed at what you find.