NPR has an interesting piece on how Production Designers work. It's worth a listen.
Screenwriters usually evolve along a certain path. They learn something about structure and story overall. This teaches them something about how a producer looks at a script. Then they figure out character -- and maybe a bit about how an actor reads a script. At some point they hit on the all-encompassing idea of conflict, and they get an insight into how a director might look at a script. This path is in no way fixed of course. And many of us either decide they've figured everything out when they get to, say, character. Or they perhaps blaze their own little trail deep into the forest, never to be seen again. Being a script consultant lets me read lots of different scripts. It's a bit like being an archaeologist -- digging up the clues to how a mind works, and what altar a writer worships at.
It usually takes a page or two at most to tell is a writer has ever actually worked on a set or not. Why? Because they are either answering that very particular set of questions or they aren't.
What kinds of questions are these? A gaffer lights a scene to make it feel deep, tragic, full of import. The DP refuses to shoot it, saying it looks like a horror movie. The two get in a tiff about just how much optimism there is in a scene. What does it really mean? They've both read the script -- and they've read it from their particular perspectives. Sooner or later the director comes in and imposes order. Sometimes this fight will recur scene after scene.
Another example: the production designer comes to you minutes before a scene is shot, vaguely pissed off that you put a coffee cup with baby dinosaurs on it in the scene. Where was he supposed to find that? Don't you realize you wasted his afternoon? He found a coffee mug with puppies on it -- and that's just the way it'll have to be.
A writer who's worked on a set understands that conceit is no abstract concept. Conceit is a strategy for unity. It's what makes a film unique. It's what keeps gaffers, actors, DP's, and even directors on the same page. It's a way of avoiding the baby dinosaur question. It's a way of avoiding endless conflict between team members. It's a way for things to go smoothly. It looks a lot like kismet.
Writers, myself included, will often not really sweat the details of a room -- especially a bathroom. After all, it's just a bathroom, right? There are enough expectations. But it's someone's job to decide what the character's bathroom looks like. And, if you think about it, you can tell just about everything about a person by the state of their bathroom.
The good news here is that you don't have to tell the production designer whether the character leaves the top off the toothpaste. But you do have to write clearly enough that they can make that decision easily and clearly.