I'm continuing my discussion of "problem" characters, and this week's almost falls into the category of "good problem to have": the scene-stealing sidekick. This is the secondary character who leaps off the page and threatens to take over the story, eclipsing the protagonist. Individual readers will always have their own preferences for characters and may like secondary characters better than the heroes, but the scene-stealing sidekick draws the attention of just about everyone who reads the book.
How is this a problem? It may mess with your plot if you as a writer are falling in love with this secondary character and becoming less interested in the hero, so you end up with a bait-and-switch that leaves readers wondering who this story is really about. That makes the story very unfocused if you establish an arc with one character and then sub in another character off the bench for the conclusion of the arc. You may get feedback in rejections like "this other character was more interesting to me, maybe you should write a story about him" or "I just didn't fall in love with your hero."
What do you do about it?
If you have a character who really comes to life as you're writing, you can change your plans and make this character the main character. This can work if you consciously do it and really revise the story to reflect the change rather than unconsciously changing mid-stream. You might be able to give the character with the strong personality the background and situation of the hero so that he can play that role in the story. It's not a good idea to try to write a whole book with a main character who doesn't interest you all that much. Follow your instincts.
Or if that doesn't work, you can raise this character's prominence and make him a co-protagonist, like maybe a buddy-cop situation. You still have the hero's arc with your original hero, but you also have this other character playing a major role in the story.
You can also make your hero more interesting. I would seldom recommend making the sidekick less interesting. The more characters in a story who jump off the page and grab readers' hearts, the better, so what you need to do is make the main characters just as interesting. A lot of this comes back to the things I said about writing the "good guy" hero. Secondary characters are often allowed more leeway while the heroes are stuck in some mold of perceived "goodness," and that keeps them from having a sense of humor, an attitude, and many of the other attributes that make a character interesting. What is it that makes the scene-stealing sidekick so interesting? Can you find that sort of thing in your main character? Let your main character have complex layers, a few shades of gray, and some good lines. Be sure that you're writing scenes that give strong conflict to your main character.
Sometimes you may need to dial back the scene-stealer to keep him in the proper perspective. There are times when too much is too much and a strong character may work better in smaller doses. I'm sure we can all cite examples of TV series in which a minor character struck a chord with audiences, so that this character was given bigger and bigger roles until he took over the show, and the show was weakened as a result. No matter how much you like a character, that character needs to be appropriate to the story you're telling.
If you're the one falling for the sidekick, make sure you're not writing a Mary Sue, a wish-fulfillment self-insertion. Make sure the rules of your universe apply equally to all characters. If your hero can't get away with a particular behavior, the sidekicks can't, either.
As I said, it's a good problem to have when a character comes to life like that. The trick is to maintain the balance in the story. Some characters do just spring fully formed to life and some take a lot of work to gradually develop them. It's worth it to do the work on all the other characters. Readers may not be able to tell which ones were easy for you and which ones were difficult.