In the first few minutes of the film Sliding Doors, starring Gwyneth Paltrow, the narrative branches off in two directions. This creates two separate realities.
In one of these, Helen Quilley catches her underground train and arrives home to find her partner cheating on her. In the other, she misses the train, giving enough time for the mistress to leave before she arrives. The rest of the film alternates between the two realities and explores how that starting point leads to two different outcomes.
The Sliding Doors screenplay is a great example of how a character’s last-minute change of plan – intentional or otherwise – can play a pivotal role in the plot. However, it’s unusual that the audience can compare and contrast both outcomes.
Another film that relies on chance is Titanic, starring Kate Winslet as Rose Calvert and Leonardo DiCaprio as Jack Dawson. In the story, Dawson wanted to return to America, and was only on board because he won tickets in a game of cards and managed to arrive in time.
As an audience, we’re left to assume that if he’d been unable to board, he would have tried to find another way to travel, and that Rose would be with her intended fiancé as the ship sank. Without them meeting, the plot would be substantially less exciting.