Start here. If you cannot name the action and the person, you do not have a movie yet — you have a setting. The logline is the same story in one breath; fill it when the subject is clear.
Subject
A screenplay subject is action plus character — what happens, and who it happens to.
Two incidents
Most stories have two hits in Act I, not one. They are easy to mash together. They are not the same beat.
Inciting incident — something happens that knocks the world off center. It is often around pages 10–15. The lead can still refuse it, ignore it, or go back to the old life. This starts the engine.
Key incident — the event the movie is actually about. It is often the Act I break (around page 25–30), but not always. After this, retreat is gone. This is the story you came to tell.
How they relate — the first creates the problem. The second commits the character to facing it. Inciting opens a door. Key walks through it and slams it. If both boxes describe the same moment, one of them is missing.
First ten pages
The opening should establish character, premise, and situation, and set the story in motion.