During the self-editing process, after you have eliminated wordiness and tightened prose, reversed passive verbs to active, trimmed adjectives and adverbs, and banished all unnecessary "wases and weres," fixed bad grammar and typos, and filled plot holes, you congratuluate yourself, thinking your work is now complete.
Well, not quite.
Set that manuscript aside for at least a week - maybe a month - and read it with fresh perspective. It's now time to cut and combine. What goes?
Any scene, exposition, or dialogue that does not fill at least one (and better if it's multiple) purpose or objective. Treat it like a lazy worker at a company. If it doesn't have a clear, defined job to do, give it one. If the scene still isn't working, send it packing.
Sample jobs: provide pertinent information, strengthen and/or move the plot, reveal character, build tension.
Just as filler words are a no-no, so are filler characters and scenes.
Can't quite bear to part with some darlings? Try combining certain characters and scenes and see if that helps. If so, great. You win.
If not, bring forth the editing knife and be ruthless.