I have been making plans to prevent this altogether when writing the sequel and have really appreciated the wealth of information and help available on line from more experienced writers. I plan to keep character worksheets for each character in the new story, incorporating not only their physical traits but also such things as family background and how that impacts upon their character and personality, what their interests are, what they like and dislike, what their dreams are, strengths, flaws and so on.
Why is it easy for character inconsistencies to creep into a story? It’s because you are inventing your characters. You are making them up out of your head. It’s a well-known saying that you have to have a good memory if you tell a lot of lies, and it is similar for created characters and stories, because it is all make believe.
The way around this is to make your characters so real so that you can identify inconsistencies yourself when you see them. Visualise your characters' physical characteristics and sketch them if you can. Study features of people you know and add them into your characters, but mix them up to make interesting composites and do the same with character traits. Imagine your characters in events and circumstances beyond the story line and work out how they would behave in those situations. Do as much of this development as you can before you start writing, but leave room for surprises and changes as the story progresses. After all, people grow and change in real life too, and it is these character changes in a story that make the fictional character real and interesting.
There is one scene in Mine To Avenge that convinces me that the central character in that particular scene has become real to me, because it evokes a particularly emotional response in me each time I read it, as I weep for the character and his family and the circumstances that led him to that point in the story, but I will have to wait until the book is out to find out what other readers think. That will be the biggest test.