Back when I was in film school, a lot of breath was expended on something called "the line." For those of you who have a better use for $80,000 than shoveling it at blowhards like me who are happy to tell you how to do what a lot of them can't seem to make a decent living doing themselves, here's a quick recap:
The line is an invisible boundary, beyond which the camera is never to venture. If it does, all sorts of bad things will happen, most of which involve an editor shaking his or head sadly and claiming there's no way to save your film.
There are a lot of complicated explanations for what the line is and where to draw it, but as with a lot of things, I find the simpler explanations better. So here's how I think of it.
Most scenes involve an interaction, and most interactions are conversations. Two people talking. That's your line.
If the camera is looking at Bob and Jeff in the wide shot, with Bob on the left and Jeff on the right, then when you get to the medium shot, guess what? Bob stays on the left and Jeff stays on the right.
When you go in for a close up, even if Jeff isn't in the shot, he's kept spatially consistent. So if Bob is looking at him, he's looking to the right.
What messes up a lot of people is that they misunderstand the concept of interaction. The apes in at the beginning of '2001, A Space Odyssey' are interacting primarily with the black monolith. So the line is between them and it.
Where this gets tricky is when there are multiple interactions in a scene. The thing to figure out is where the main interaction is and let that drive the line.
Let's say Miss Rutkowski is confronting a room of unruly third-graders. One end of the line goes straight through Miss Rutkowski, while the other end goes through the room of 8-year-olds. When Billy and Steffie pass a note, their line is between them. But you know how the line falls between them because you've established their screen relationship in the wide shot with Miss Rutkowski. So if Steffie is on the right of Billy in the wide shot, you keep her on the right when you come into the two-shot.
I know what you're thinking. You're thinking, "But Brian, I want to cross the line. I want to shoot Steffie and Billy passing the note from behind, with the note in the foreground and Miss Rutkowski in the background."
So do it.
This whole line thing is just a way to keep from jarring your viewers. Keeping the main interaction consistent makes it easier for your audience to follow the story when you make a cut. If you have someone looking to the right in one shot and then to the left in another, your viewers are going to have a harder time following what's being said.
So if you want to shoot the note close up in the foreground, it's probably because you want to emphasize the note. You instinctively feel that the image of the note being passed is more important than the dialogue being said.
I'm okay with that.