Character development is one of the most complicated aspects of writing a novel. Too much back story and the reader feels insulted or, worse, bored. Too little, the reader’s lost.
Neither do you want the reader to muse, “methinks he doth protest too much,” so don’t overdo it.
One of the best methods I’ve found to test a character(s) believability is to have an actor read my material. If they can “act” the character with the information available, character development is probably on track. On the other hand, if you get a “lost” reaction, it’s time to return to the keyboard.
In my recently completed novel, The Will of Grace, about the Book of Kells, there’s one particularly nasty fellow. He originally showed up around halfway through the novel. A friend, who is a New York City stage (as opposed to film) actor, read the manuscript and bluntly told me she had no clue how she would act that character. She wanted more back story. And she wanted it earlier in the novel. That was a significant light bulb for me. Practically a siren. I had a certain discomfort with this character, in terms of how he was written. I couldn’t identify the issue until Erin read through the manuscript. There was too little back story. The audience could not identify with him.
After a rewrite, that character now shows up quite early. The reader meets him during his childhood and learns his story through adolescence and into adulthood. When he shows up as a bad guy later in the novel, the reader now understands him, where he came from, and what his motivations are. This is crucial for the reader.
We cannot just drop a character into a novel and expect the reader to understand him or her. We might, because we know the history. But that’s information the reader does not have. Give them something to help understand the character. I don’t mean ten thousand words. Perhaps a few gestures, phrases/expressions and/or one minor scene for a contextual understanding are all that’s needed. When the “bad guy” shows up, the audience needs at least a minimal understanding of who he/she is.