You need to consider your characters outside the plot as well. They'll have likes and dislikes that have nothing to do with the story, funny mannerisms they're unaware of, things that set their teeth on edge, irrational fears. The strangest things will get them into certain moods.
Let's take a real person, Charlie (name changed to protect the not-so-innocent), as an example. Charlie hates M&Ms - they make him sick, after he ate too many before a long car ride and produced a bag full of multicoloured puke. He's terrified of worms because when you chop them in half, they still wriggle about - but spiders, cockroaches, and other bugs don't really bother him. He gets bloated after big meals and can't stop burping. He can't stand it when people sit on his bed with pants they've worn outside of the house. He hates afternoons because they put him in a lethargic, melancholy mood.
Were he in a story, it's likely none of this would be directly related to the main plot or his motivations. But it's the little things that people say or do or feel that bring them to life. And when you get them down, you'll see them appearing in your story quite naturally. For example, Charlie may be having some friends over. He may turn down the M&Ms they've brought, and keep them out of the bedroom so no one sits on his bed.
It may also be useful to do some online personality tests, as they'll get you thinking about how a character would react in various situations, and how they'd respond to some tricky questions.