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So you have things planned out to the last detail - or if you're a pantser then you have an idea in your brain. It might be a bit fuzzy, but it's there. Either way you now know your MC and you're moving forward when all of a sudden one of your supporting characters decides to steal the show. Chances are this character is lots of fun with a dazzling personality. It's very tempting... but you have a story to tell and it's meant for your MC!
What do you do?
There are a couple of things that might be going on here. The first possibility is that your MC is falling flat and your subconscious is reaching for something easier. AKA the shiny, fun character that feels natural to write. If this is the case (and only you know if it is), you should stop and reassess your character building. Do whatever it is you need to do to get inside your MC's head. Try to find out what's preventing her from coming fully to life.
BUT if the reason is that the supporting character is just one of those show-stealers, I recommend sitting down and having a heart-to-heart with him. What? Is my insanity showing again? Seriously! I've done this. And what I said was, "Listen, this is X's story, not yours, no matter how much I love you. So if you behave and anything ever comes of this, I will make you the star of the next book."
In other words, give yourself permission to explore this character that's so grabbed you, but in such a way that it doesn't overshadow the wonderful work you're currently pursuing.
Good. That sounds far more sane. Um, I mean reasonable.