All production design is important, but in any given scene, it's likely that actors interact with chairs the most.
You might never notice them, but they're often doing a lot more than you think. In this exceptional video essay from Tony Zhou, he explores just how much of an effect these mundane set pieces can have on a film and its characters:
This is one of those video essays where you really need to give it a chance and let it play out. It seems totally ridiculous at first, until you start to realize just how important a chair can be in telling the audience how to feel about a particular character or how a character feels about themselves. The examples from Chaplin's The Great Dictator are particularly fantastic, and not only are the chairs an essential piece of production design, but they also become props that serve as a source of comedy:
Chairs are obviously not the only production design element that can greatly affect a scene or a character — clothing probably plays an even bigger part and has a larger psychological effect, but it's also a more obvious element. How often are you really thinking about the chairs in a scene? If you're a production designer or you work in the art department, probably a lot, but for the vast majority of people, they tend to go unnoticed, and this video essay is a great reason to pay a little more attention the next time you're watching a film or sitting your actors down in a scene.
Be sure to check out Tony's page for more terrific videos.