In my last book MIND THEIF I struggled with one character, Debbie. She was an interesting character with a very clear motivation that wasn't shown until her final confrontation, with a lot of reworking her I had her I had her stay true to character except when she slipped up. But the scenes with her were painful.
Naturally I blamed her for the horrible scenes until I put the manuscript aside for a month and looked at those scenes again. Once I had a small degree of objectivity I could see she wasn't the one ruining the scenes it was my main character, Howie.
He had a clear, if selfish, reason for hanging out with her. But not strong enough to excuse her little slip ups, so she either made him look unbelievably naïve or that she was an unbelievable character. The problem was I was showing their interaction but not telling what my main character was inferring from their conversations. Oddly enough I had the problem of showing not telling.
When someone says something to a person two things happen, they hear the words and they infer the meaning. It's what the person infers that is the most important. I had my Debbie slipping out of character on purpose and my main character was barely reacting to that. That's normal, if you questioned every slip of the tongue that someone has in a conversation you wouldn't have many friends. But you would think about those odd slips of the tongue.
In making a character come to life, it's not enough just to have them act realistically but the other characters have to react them. Even if it's only internally they will think about what is being said to them and what they think about what is being said is at least as important as the actual words.
By showing what Howie is thinking about Debbie's little slip ups it brings both their characters to life. As he is now reacting to her even if only in his head.
It turned out my problem wasn't with the actions I was describing, it was the reactions that I knew my character had but I wasn't describing.