I tend to do a timeline with a few pics to show the scene at key points. The timeline shows how many seconds for each shot, with a text description. If any shots are over about two minutes I have a close look at whats going on.

So

30 sec title and copyright threat

3 min Intro, scene and characters

3 min Oh Noes ! the problem

5 min Good greif ! the solution

3 min Happy ending - and

30 secs end credits

total 15 mins

A short film has the advantage that it has to move along and not dwell on anything.

If you are doing a full movie you can have rwo characters gaze into each others eyes and kiss for 5 minutes , at which point all the guys are out getting popcorn :)