behind the scenes

I wrote a thousand words last night, a whole scene, that I know will never be in the book. The book is told in first person, and this was a scene my MC will never be at. But it's a turning point for a secondary character, and I felt I needed to see it happen. First time I've done this sort of thing, but I think it helps to understand her more.

How about you? Have you ever deliberately written something behind the scenes that you know won't make it in?