Flashback
When writing scripts, Syd Field says that you should only use flashbacks when there is no other way to give the facts to the audience. Flashbacks should happen in response to a character’s emotional state. They have been triggered by something in the present that reminds them of an extreme emotion in the past.
The rule of only using flashbacks when there is no other way reminds me of a similar rule about dialogue. Screenplays are about telling stories with pictures. Only use dialogue if the information cannot be got across with pictures alone.
alex4D old blog 
Syd Field, last time I checked, hasn’t written any screenplays. And while I strongly believe that you don’t have to be the best in your field to write or teach about it, I take all of these script gurus with a large boulder of salt. They all have a tendency to make broad generalizations, disguised as established rules.
This flashback rule, for instance. I understand where it’s coming from — flashbacks, like voice over, are often created as a crutch for bad shooting or writing. It’s an editorial device to solve other problems (and what is wrong with that by the way?). But the rule? I wonder how Field talks about MEMENTO, for instance? Flashbacks can be a style choice, they’re not always about fact distribution.
I think he’s attempted to write scripts over the years. His training comes from the point of view of the reader for the producers. His books let writers understand what standards the gatekeepers will use when assessing scripts. The first stage of selling anything is to research those who you plan to sell to. In this case Syd provides the vocabulary of the film development community.
One of the few writers of screenwriting books who also successfully sells screenplays is Blake Snyder. He’s written two very good books on script structure.