(or at least plan it)
I was having trouble fleshing out a character in my WIP. He was irritating and mean but two-dimensional. I added and subtracted dialog for a while, but it didn’t help. Then I started outlining the sequel. (I doubt I’ll ever write it, but I typed up some ideas.) Suddenly my flat character had a past and a future. His actions had motivations. I snuck a few bits into my WIP and so far, it’s working well.