In screenwriting, the Catalyst occurs about a quarter of the way into the story and involves the protagonist receiving crucial information that will change his or hers orginal desire into a specific goal.

Usage

About a 25% into a script, the protagonist is going to have to make a decision as to whether or not to take the journey to acheive his or her goal. If the protagonist does not take this step there is no story. So, in essence, it is at this point, when the protagonist receives information that will serve as a Catalyst to give the necessary push into the journey and toward the goal.

Note, the protagonist will be pushed to just before this point by various Nudges, but it is the Catalyst that will get the ball rolling.

Also, it is during the Catalyst when the protagonist will formulate a plan as to how the journey will be taken. In a Two-goal Plot, this plan will take the protagonist toward an Initial Goal to the Reversal, which will then force the protagonist to create a new plan towards a New Goal.