Every movie has a lesson, whether it's noticeable or not. Without a theme, there is no purpose to the film. When you're writing your script, you want to make the film meaningful to the audience so they have something to carry with them once the film ends. Using the beat sheet method, the theme is usually on page 5, or five minutes into the film. The theme is something that the protagonist has to learn in order to develop as a character. The theme doesn't take full course until the end of act 2. Here's a few ways you can have a theme in your script.
1: Have a character tell them the lesson they need to learn.
Usually in most films, a character will tell the protagonist what they need to do to change early in the first act, once we can acknowledge the protagonists fatal flaw. The fatal flaw is what holds the protagonist back from developing as a character and prevents them from reaching their goal. Usually, the theme is used in a if/then statement. For example in "The Hunchback of Notre Dame" (1997 animated version) the theme was "If watching is all you're gonna do, you're gonna watch your life pass by without you." In other words, it's like saying "If you keep doing (insert fatal flaw) then you're going to (result of fatal flaw). Of course not every theme is going to have an if/then statement.
For example, let's take a look at Grosse Pointe Blank. Early in the film, we learn that Martin Blank, played by John Cusack, isn't quick on his feet. That's the character's fatal flaw. After the first action scene in the film, Grocer, the antagonist played by Dan Aykroyd, tells the protagonist, "Life is full of second chances." That's the lesson Martin Blank has to learn throughout the film because of his fatal flaw.
2: Show the theme
Although it can be hard to identify the theme in subtext, the audience might get a better idea of what the theme is, even if it isn't specifically said. For example in "The Princess Bride" the story the grandfather is telling his grandson is told within the story, The Princess Bride. So according to the book the grandfather is reading, we learn the theme revolves around love and it explores the idea of true love and the things we do in the name of love.
3: Have the protagonist mention the theme.
Normally, the protagonist doesn't mention the theme of the film early on, but in the case of Forrest Gump, most of the film is told through flashbacks so it's okay for Forrest to mention the theme out loud as he tells his story. What was the theme? "Life is like a box of chocolates, you never know what you're gonna get." It's powerful and the theme is easily shown throughout the film.
Side note: If you have an ensemble cast, like "Ferris Buller's Day Off", the theme doesn't have to be mentioned to the protagonist, because the film follows three friends who take a day off from school. At first you'd think Ferris is the protagonist, but the real protagonist is Cameron because he hits all of the beats in the beat sheet. Early in the film, Ferris breaks the fourth wall and tells the audience, "Life moves pretty fast. If you don't stop and look around every once in a while, you could miss it." It's powerful and it's something Cameron has to realize during the film.
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