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Act I sets the tension and gets the hero up the tree by establishing the characters, the story world, the story concept, and the conflict. We learn who the protagonist is and what he wants. We become familiar with his world before it gets disrupted, and we are encouraged to like him or feel sympathy toward him. We also learn where and when the story is taking place and are introduced to the rules of this story world.
An event must happen in Act I to disrupt our protagonist's life, eliciting action from him and setting the story in motion. This is the inciting incident, and it often occurs early in the first act. The first major twist or reversal in the story occurs at the end of Act I. The protagonist has experienced a major change in his life and has made an initial decision about how to react to this change. The conflict has been established.
The first act of Thelma and Louise (written by Callie Khouri) introduces us to the two protagonists and their normal world. Their goal at this point is a weekend away, a bit of an escape from the daily grind. For Louise (Susan Sarandon), it's a break from waitressing and an opportunity to get her mind off Jimmy, her non-committal boyfriend. For Thelma (Geena Davis), it's a chance to escape her domineering husband. When they stop at a bar for a few drinks, we experience their bond and their sense of humor, and we quickly get to like them. The inciting incident occurs when Louise shoots the man attempting to rape a drunken Thelma in the bar's parking lot, disrupting life as they knew it in a dramatic way. By then we're on their side, ready to follow their adventure.
Generally speaking, Act I covers the first 25-30 pages of the screenplay.