Every object in a film frame was put there.
This sounds obvious. It is consistently forgotten in the moment of watching, and almost never mentioned in the moment of criticism.
The room your characters are in - every surface, every piece of furniture, every object on every shelf - was the product of a deliberate decision by a production designer working in consultation with the director, the cinematographer, and the costume designer to create an environment that was doing specific formal work in the film. The work that environment is doing is almost never described in film criticism.
WHAT PRODUCTION DESIGNERS ACTUALLY DO
The production designer creates the visual world of the film - everything that exists in the frame that isn't the camera, the actor, or the light. Sets, locations dressed to specification, props, the colour palette of every environment, the specific texture and age of every surface. For a studio production, the production designer leads a department that may include hundreds of people. For an independent film, the production designer is often doing the work of that entire department with a fraction of the resources.
The production designer works from the script but is not constrained by it. The script may say 'interior apartment - night.' The production designer decides what the apartment looks like, what it tells you about the person who lives in it, how it relates to the other spaces in the film, what colour it is relative to those spaces, and how that colour relationship shapes the emotional register of the scenes set within it.
THE PARASITE ARGUMENT
The Parasite production design - led by Lee Ha-jun - is the most discussed example of recent production design that is doing the film's primary formal argument. The Park house is not just a beautiful set. It is an argument about class as spatial architecture. The glass walls that expose the interior to the garden (and the garden to the interior, and the street to the garden) create a visual system about visibility and privacy that maps onto the class system before the film's dialogue makes it explicit.
The specific object that does the most work: the light well. The narrow vertical shaft that connects the underground space to the garden level is, in the final act, the formal image that makes the whole class architecture visible from above and below simultaneously. That wasn't in the script. That was in the design. The design earned the ending.
THE WELLES EXAMPLE
The production design of Citizen Kane - led by William Cameron Menzies alongside Gregg Toland's cinematography - is the formal engine of the film's argument about power and decay. The scale of the Kane interiors grows as Kane's power grows and as his human relationships contract - the ceilings get higher, the spaces get wider, the furniture gets further apart. The architecture of Xanadu is the architecture of a man who has substituted accumulation for connection.
The famous low-angle shots that show the ceiling - unusual in 1941 when most studio films were shot in ways that avoided showing ceilings - are only possible because the production design built ceilings into the sets, which real locations have and studio sets usually don't. The formal choice (low angle revealing ceiling) and the production design choice (build a ceiling into the set) are the same argument from different departments.
WHY IT GOES UNCREDITED
Production design goes uncredited in film criticism because the successful production design is invisible. The environment that fully succeeds reads as real - as simply the place the film is set, not as a formal argument about the film's subject. The work of the production designer, when it works, disappears into the experience. The cinematographer's work is visible as photography. The editor's work is visible as structure. The production designer's work is visible as reality. The criticism that notices it requires deliberately questioning the apparent reality of what you're seeing.
The production designer's work is visible as reality.