So atmosphere is weather, and even background sounds. But what do you do for atmosphere if you’re indoors? Well, first, you can make the scene atmospheric by adding fog. No reason not to add fog to any setting, indoors or out.
Or you can make it dark, so that most of the details are obscured and hard to see. There’s some atmosphere for you! (I checked out Alien, which was the #2 atmospheric movie on my list, and it was both dark and foggy.)
But what do you do if your scene has to be both indoors and brightly lit?
Hang up some posters.
Like the lip-syncing scene in Pretty in Pink. Or, magnificently, the museum scene in Ferris Bueller’s Day Off.
Okay, it doesn’t have to be posters. But put something on the walls–unless the indoors has to be deliberately blank, as in Let the Right One In, in order to contrast with the (highly atmospheric) world outside. Or make the walls themselves interesting, as in The Shining.