How can you tell when flashbacks are unnecessary?
The question goes to the heart of the matter: who am I writing for and where is his or her sweet spot?
Any time you veer away from telling the story that is taking place, you need to test against your own inner reader whether or not the flashback or the digression or the venture off into this or that is going to pall on the reader and make him or her antsy or cause him or her to start scanning in hopes that the story will soon return.
How do you judge this (and it is a matter of judgement)? A big part of learning to write any kind of copy, or indeed creating any kind of craft work or art that you would like someone to be willing to pay money for involves developing your own willingness and ability to put yourself into the shoes of your reader, and developing your own inner balance wheel and associated techniques for deciding whether whatever-it-is works or doesn’t work.
What, you thought you would just sit down and rattle off a best-seller? You thought learning to write fiction would be a lot easier than getting through law school?
By associated techniques I mean the crafty things like bringing the tension to such a pitch that the reader, in anticipation of that tension being resolved, is willing to put up with what she hopes will be a temporary delay of some kind of impending gratification, and stuff like that.
And of course making sure that the flashback or whatever is in and of itself something of interest and interestingly conveyed.
The most fundamental way to assure yourself that a flashback “works” for the reader is to make sure that it’s something your reader wants to know, as opposed to something you the writer want the reader to know. What the reader wants to know is the channel where the river wishes to flow.