The more you can delay the writing the better it will be. The longer I wait the more problems I've solved, instead of pouring my energy into writing something that isn't structurally sound. I like the freedom of being sloppy at this stage and not having to do great writing. You can't play with it the same way. He builds each scene to a point where an action reveals something about the character.
Reaction can show something about a character, so use that wisely.
Lots of people who will read your screenplay skip over the action and only read dialogue, so your dialogue better be great, too. The great characters have some mystery about them. We don't always know exactly what they're thinking or what they're up to. You have to leave some room for the audience to do some thinking on its own, without everything being spelled out. This gives a movie its life and vitality.