Does a movie's mise-en-scène happen by accident?
If not, what or who determines it?

A movies mise-en-scène is the movies arrangement of scenery and stage properties in a play. So no, it doesn't just happen by accident, a production designer or art director works in collaboration with the director of a production to create a set for a theatrical, film, or television production.

c. gabriele was a catholic priest while also being a composer.