Containing a story within a single location is a great way not only to spare your budget undue inflammation, but also to bring tension and drama to an unavoidable and concentrated pitch. That said, containing the breadth of a story within the confines of a single setting does present its challenges.
Here are a few tips on how to write single location films:
Dynamic and opposing characters are key. Try to limit the number of characters involved in your story, and to emphasize their differences. If two characters of the same mind and temperament are narratively chained together for the duration of the plot, keeping them in one place multiplies the chances of things devolving into a snooze fest. But if these characters have dynamically opposed outlooks on life or whatever has befallen them over the course of the narrative, trapping them together only intensifies the conflict driving your film. Be sure that your characters are distinct, even if they wind up in agreement or have a similar background. Variety is the spice of single location films, and it must flow.
Try to incorporate geographical or atmospheric changes into the plot. The characters may be herded into a single spot, but that doesn’t stop you from getting creative with their surroundings. Choose an appropriate moment for the weather to take a turn, shuffling the players inside, or raising their voices to accommodate a thunderous storm, for instance. Maybe there’s a heatwave that settles as the night goes on. You have one location, keep it alive.
Take advantage of the paired down scale. When everything hinges on talk, and the props and set decorations take on meanings through their ever-presence, they take on the potential for meaningful action as well. Leaving a scene is pretty mundane, but if everyone has been in the same house for the entirety of the film, and then a character bites the bullet and charges out the door, that has the potential to feel monumental. When there’s so little to work with, small gestures take on greater significance. So too, does absence.
Punch up the dialogue as much as you can. There is less room for humdrum realism in a single-setting story; the dialogue must sparkle and keep the viewer entertained where the location remains static. Think of screwball scripts like that of His Girl Friday, largely confined to a newspaper office once its plot kicks in. Conversationally, the leads of His Girl Friday are nothing like real people, but they shine thanks to their superhuman chatter, pacing back and forth in the same headquarters while the story they cover develops outside, and a world unto its own presents itself through their tete-a-tete.
In short, avoid redundancy, incorporate as much change and conflict as possible, and keep the golden rule for a single location film in mind: keep it entertaining.