When writing a script, always look for ideal moments to insert dramatic, comedic or expositional lines designed to make great sound bites for trailers.
It pains me to say it, but the marketing gurus are right. A great trailer can be the difference between a hit and a flop, so make sure it has all the sounds bites it will ever need. Here are some samples of great dramatic trailer lines from over the years:
"The end has come." - The Lord Of The Rings: Return Of The King
"Why so serious?" - The Dark Knight
"Soon we must all face the choice between what is right and what is easy." - Harry Potter and the Goblet Of Fire
"Every second we stand here is a second lost!" - Valkyrie
"We have all our rotten eggs in one basket. The objective of the operation? Blow up the basket." - Inglorious Basterds
All of these lines serve a different purpose. One is to give a sense of finality, another to convey a chilling madness, another an ominous feeling, another extreme tension and another to sum up the film in a punchy nutshell. They don't just work in the trailer, either. The good ones work great in the film too.
It must be said that this also applies to the visuals. A script should provide some interesting visuals to explore or unique scenes that naturally intrigue and invoke curiosity.
I highly recommend checking out all of the above trailers as they are full of excellent examples.
"For further reading on the topic, this article might be helpful."