I have a crush on my fiance's best friend
So you've made me curious and now I'm wondering if you plan on doing something about it?
I don't know. I think about them all the time.
It's not unusual to crush on someone besides your fiance, but you shouldn't act on it. That will devastate your fiance, and probably not work out with the friend, either.
I won't be able to keep quiet about it, I don't get how I need to be since I'm attracted to him.
You should just move on from this and not act on it, do not speak to them anymore and move on with your partner. You've chosen someone to love already and if you do love them then the kind thing to do would be cutting contact with their best friend and explaining to them the situation in hopes that they understand.