According to Linda Seger in MAKING A GOOD SCRIPT GREAT, the catalyst is an event that starts the story and begins the action. Once we've set up our main characters and their world, the script needs some event, action, or information that pushes us into the plot. She writes that there are three types of catalysts: action, information, and situational with an action catalyst like a murder (Witness) being the strongest. Finally, in structural terms the catalyst happens by page 15 in a feature screenplay. In a TV episode or short film it might happen very close to the beginning. Anyhow, anyone want to expand on this or give examples from recent films?