Believe it or not, every film contains three key elements, the story, character, and theme. Without all of these elements, you don't have a film.
Story
Story is the plot of the film. Every film tells a different story to its audience and contains conflict and stakes that are continuously raised throughout the script.
For example, Titanic follows a tragic love story between a lower class man who falls in love with a rich woman on the famed "Titanic" ship that tragically sunk in 1914.
"The Wizard of Oz" follows a teenage Kansas girl who ends up in Oz, and the only way to get home is to meet the wizard.
Think about how boring it would be to watch someone easily achieve their goal. I'm sure "The Wizard of Oz" wouldn't have been made if Dorothy automatically clicked her heels when she got to Oz.
Character
Characters are an essential element to a film because they are part of the plot. From a writers perspective, characters are the heart of the story. Whenever someone watches a film, they want to see someone face challenges during their journey, yet at the same time, they also want to see someone who they can relate to on screen.
What are their strengths and weaknesses? Positive traits? Negative traits? Fears? What are their hopes and desires?
It's also important to note that your character, mainly the protagonist, likable early on because your audience wants to root for them to succeed in the end. If your protagonist is a jerk and has unlikable traits, the audience isn't going to feel satisfied and lose interest quickly.
In The Truman Show, for example, Truman spends most of the film making sense of his "reality" and trying to escape from it. Not only is he trying to escape, he's spent most of his life unaware that his life has been broadcast to millions of people around the world.
Throughout most of his life, Truman has always wondered what the outside world was like, but due to the "reality show", he's only familiar with the town he's lived in. Later, when he ventures out to escape to the real world, he faces various challenges that prevent him from leaving, and later realizes the only option left to escape is to face his fears of the ocean.
Theme
The final, and most important element, is theme! By now, you already know that the theme is the message of the story and the lesson you want the audience to walk away with. Without having a theme, there is no point and the audience will walk away feeling unsatisfied.
Believe it or not, every film has a theme, whether it's obvious or not. I've heard writers talk about themes and some say that most films don't have a theme or shouldn't have a theme, to which I disagree.
Just because a viewer can't identify the theme of a film, doesn't mean there isn't a lesson the writer wants to share with the audience. Who would Shrek become if he spent most of the film blocking people out and spend the entirety of the film isolating himself from everyone?
Like I've mentioned previously, the theme can be obvious, such as "Life is full of second chances." and other times, the theme isn't obvious at first, like in When Harry Met Sally, where the theme was "Can a man and a woman be friends and nothing more?"
500 Days of Summer is a perfect example of where the audience has to dive a bit deeper into discovering the overall theme. Near the beginning, we get a sense that the theme is "Expectations vs Reality". If you think about it, the first half of the 500 days was the expectations, but the second half of Toms relationship with Summer was the reality.
Consider for a moment that your script is like a burger (or unless you're vegan, a veggie burger). The story is meat of the story. The character and theme, on the other hand are the buns that hold the meat together.
If you have any questions, feel free to let me know!