Caution, this complication is reversing…

“This script needs more reversals in Act Two.”

An easy criticism to make of most scripts, but what does it mean? The word ‘reversal’ is so strong that I used to think that it meant ‘a major change of fortune.’ But when used in writing, it merely means an obstacle for the protagonist of the scene.

If the scene is about your hero wanting to smoke a cigarette, the reversals he faces are a series of increasingly difficult obstacles he faces to achieving his goal. Firstly he doesn’t have any cigarettes, then having got a cigarette his matches are wet, then having got a light from a stranger, having started to smoke he discovers that he isn’t allowed to smoke in the bar where he is sitting, going outside he bumps into the woman of his dreams who thinks he’s given up smoking…

From The Understructure of Writing for Film and Television:

The development of a scene is made up of several reverses leading to the crisis that, climactically, your protagonist must resolve.

Complications (as in ‘complications arise when…’), on the other hand, are crucial turns in the plot:

A complication is a development in the process of events that affects the overall objective of the protagonist…
…A reverse can never be good, while a complication in itself can be neutral: it is how your protagonist handles the complication that is important.

For example, there’s nothing bad about James Bond falling in love, but it adds complications to his mission, and his reaction to falling in love is what matters from that point on in the story.