HB is an American woman who has had 30 years of experience working in the theater industry, specifically in tech, props and production management.
When an actor in a stage play is “really overacting and they’re playing to the back of the house”, one would say that they’re “chewing the scenery”. This could be a good thing or a bad thing. If the role calls for that amount of energy, then it’d be a positive thing, but “generally it’s kind of negative”.
“That scene was exhausting to watch, it was a scenery-chewing performance.”
“He’ll be picking scenery out of his teeth for days”
The actual meaning of this saying is a criticism of actors that don’t take realism into account during their performances and just focus on expressing as much emotion as possible. Especially in theater performances, it’s common for actors to need more energy in their acting since they have to portray to audience members that are seated far away from them, but this saying seems to acknowledge some threshold for the accepted amount of exaggeration in acting. This showcases the need and appreciation for realism, or at least a balance between realism and emoting within a performance.
The imagery of the saying also suggests a dynamic between actors and stage hands. If a performer’s acting “chews up the scenery” or, in other words, destroys the stage, and is seen as a negative thing, then it’s possible that there is a need for respect between actors and those who work backstage. If an actor’s performance is overwhelming in action, it’s possible that the audience would overlook the hard work of the stage hands in making the image on stage come to life, which would make the stage hands upset.