Writing, at least the initial stages, is fun. We create characters we adore and have them do things that delight us. If we love research, we write scenes that show off what we’ve learned.
Then comes the hard part—editing. An important question to ask, as we go through the revision process, is: What purpose does this character and/or scene serve? How does it advance the emotional arc of the story and/or how does it advance the plot? Ideally it does both.
If you don’t have an answer—give the scene or character a purpose for being there. There is almost always a way to do so. If you can’t, then take it out, but only as a last resort and do NOT throw it away! Keep a file—hardcopy or on your computer—of scenes or characters you remove. It’s possible that you will realize, as you go on, that the scene or character was important after all.
Note: I strongly believe that the ideal strategy for revising a story or novel is to do a quick read through NOT making changes but rather making notes about changes you THINK you will need to make! When you get to the end, look at your list. Which is the most important change? Concentrate on that change—NO MATTER WHERE IN THE STORY IT OCCURS! Every change you make will have a ripple effect on the story. If you make the most important (global) changes first, you will end up doing a lot less work and some of the smaller issues may disappear once the global change is made.
So remember, every character, every scene must serve a purpose both in the emotional arc of the story or the external plot or both.