I think flashbacks are a great idea, but some people do not. In The Two Towers Extended Edition`s Appendices, someone (I don`t remember who it was, probably a publisher) told Tolkien that he should not use flashbacks in story telling (He was refering to Merry and Pippin`s story about the flooding of Isengard), but Tolkien did it anyway. I think it is just fine, as long as there are 2 or more groups, or if someone is recounting something that someone said or did. Did that make sense?
