Actors become very professional and proficient about watching out for each other's light and not stepping on each other's lines.
It's interesting the kind of freedom the musical form gives you. The rules are out the window. You can get impressionistic without seeming pretentious. Because it's perceived as an inherently accessible form, it gives filmmakers some leeway.
This is going to sound ridiculous, but I remember watching 'Boyz n the Hood,' and there is a scene where Cuba Gooding, Jr. gets pressed against a car by a police officer, and he starts crying because it's so humiliating. I remember thinking in that moment that I could totally identify with him, and I'm a little white kid from Canada.