There are plenty of mistakes to be made when it comes to crafting convincing characters but I’d say the biggest one is a lack of consistency. If a character acts a certain way in one scene, it’s important that she acts the same way in another. Unless, of course, there’s a good reason to explain the change. But if you establish early on that your character is afraid of heights, then have her calmly walking across a tight rope a few chapters later with no mention of that phobia, you’ve got a problem.
Consistency is key when it comes to personality, preferences, manner of speaking, background, and a thousand other details. You can’t ever drift when you’re writing a character. Each time you need to write a line of dialogue, get in that character’s head. Don’t put words in his mouth, let him say what he would say. If he was real and actually in the situation you’ve put him in.
Now, when it comes to major characters, this is usually easy. We writers often become very attached to our main characters, spending hours developing every facet of their personalities and backstories until they feel as real as our best friend. For some of us, maybe they even become our best friend. Writing such characters is easy, natural, because we don’t have to imagine what they would say or how they would respond. We know them so well, that every line virtually writes itself. The challenge arises, and many manuscripts fall apart, when we go to write less important characters.
You know the kind I’m talking about. The ones you didn’t spend much, if any, time thinking about beforehand. The ones who are mainly there to fulfill some role that’s necessary for moving the plot forward. The general recommendation, of course, is to avoid such characters entirely. But when you just have to use one, make him consistent. Down to the small details of the way he walks and the way he talks. When you do this, even for characters so minor as to be insignificant, all of your characters will feel more real. Your entire manuscript will take on new life.
So remember, consistency matters.