Doors. They're so ubiquitous in our everyday lives that it's easy to be unaware of just how big of a role they can play in storytelling.
They can act as barriers, pathways, and visual representations of the relationship between two characters, something video essayist Darren Foley of Must See Films explores in yet another excellent cinematic study.
The name of the game in film is "show, don't tell." So, the more tools you know you have at your disposal within your diagesis, the better -- and doors are certainly things you shouldn't overlook. Like Foley explains in his video, they can become visual representations of a number of things, the status of a relationship for instance. In a film like Monsters Inc., doors play a dual role as both literal and metaphorical gateways into the lives of children. At the beginning of the film, Sully uses a door to enter Boo's life as a monster, but later on uses it to enter her heart as a friend.
There are countless ways to use doors as a narrative device -- to communicate power/weakness (who's allowed in and who's not), to communicate emotional/mental states (open doors vs. closed doors), and to elicit a few laughs (The Dude barricading his door the wrong way). Understanding their potential for storytelling will definitely help you tell new and challenging stories.