A few tips I’ve learned over the years.
First, Re-Read your stuff…That’s a given. Not only for spelling but sometimes I find reading a chapter (OR chapters) you tend to find yourself adding additional scenes or lines of dialogue which isn’t necessarily a bad thing and can even help with your narrative especially if you want to go in a specific direction.
Not only as PoM said let the characters take a life on their own but if you’re like me and you’re doing a crossover set in a specific time (Example a Crossover set during Season 1 of ONCE) never be afraid to deviate a bit from the original story even to the point in deviating a lot, if you can’t find a way to add one set of characters with the other then don’t, just say (A.B.and C happened in the background while one set of characters/ The One’s you focus on in the story did this thing.)
I tend to find (At least for me) writing Dialogue is the easiest, I myself tend to tell a story through dialogue. Never be afraid to do that especially if you are a character driven person.