I’m not a big fan of the flashback as a narrative device, but there are times when it’s appropriate.
If you’re going to shoot a flashback, the key is to make sure your viewers know that’s what it is.
The easiest way to do that is to follow in the footsteps of the directors who have gone before you. Film grammar has evolved a particular device that viewers understand to mean “here comes the flashback.” Basically, you start on the character’s face who’s entering reverie and flash to white over the course of three or four frames, then cut to your flashback scene.
Sometimes it helps to come out of white for three or four frames into the flashback scene, too.
There are a couple of other devices that help, too.
One is to use a camera move that’s consistent across both shots –– usually a push in –– so it feels as if the camera’s movement is continuous. Another is to use a continuous gesture on the part of your character. A head turn is always good. Have your character make the same gesture in the same framing in both shots and you can edit the two together as if it’s a continuation of the same gesture.
The look of the film can help communicate the temporal relationship as well. For some reason –– probably because papers yellow and old photos were sepia toned –– people associate warmer tones with the past. So it often makes sense to use those tones in the flashback as subtle clues that what you’re looking at is older.
Want a quickie demonstration? Take a look at ‘The Love Guru’. No, I’m not kidding. Marco Schnabel missed the boat on a lot of stuff in that one, but he did do his homework on the flashback thing.