Except when they’re in the bathroom.
When writing a script, keep track of each character, and always try to know what they’re doing at any given point in the narrative, even if we never see it in the script or in the movie. Doing this helps keep the timing right, and the rhythm of the movie benefits from it.
I try very hard to make sure that I track each character. Sometimes I use colored cards, or symbols on my scene cards. It’s easier when they’re all laid out on a table. I also usually start writing scene cards by character, so I know how each character flows through the story. I don’t always write every scene card, but I end up writing most of them.