Most films are shot out of sequence and continuity is an issue to be taken seriously.

You should have someone (the continuity supervisor) who takes Polaroid snaps of actors, locations, etc. to ensure the continuity of makeup, costumes, arrangement of props and set pieces, the level of water in a glass, and many other things.

There is also the issue of continuity in acting – instruct your actors to be consistent with sipping drinks, smoking a cigarette and other actions. In other words, if an actor is doing a smoke-and-talk scene, instruct her to take puffs at specific points during the dialog, and to repeat the timing for every take.

This may seem excessive, but I guarantee you will be glad you made the effort when you edit the movie. If the actress takes sips from her glass at random times during the scene, cutting from one shot to another with no continuity errors may prove very tricky indeed. She could be drinking in one shot and when you cut to a different angle, she could be taking the glass away from her mouth – an obvious continuity error. You’ve been warned.