
Remember, the theater play is a group work. Currently speaking, the coach/teacher knew that Maru is already a lot more talented compared to his colleagues, hence even if they ask his colleagues to run up to Maru's pace, they'll just be discouraged and compare themselves to Maru when in fact, they're doing great at that pace of them.
What the coach was saying was not to "not" stand out (as Maru can't really help but to stand out) but for Maru to not hog the spotlight with a simple misstep. Maru, too, knew that. That because of his excitement, his theater group also kind of broke out of their characterization and also felt his excitement and so he accidentally made people to focus on him rather than their group because his character was bound to have that feeling when his theater play group shouldn't have that feeling. If Maru's mistake was letting that excitement of his be felt by his group, his group's mistake is getting carried away by Maru's excitement, and the teacher/coach knew that Maru could make it so that they couldn't feel that, hence that.
I hope you get what I mean...
Plus I think that self-absorbed guy acted quite badly. He's too tensed.
Yaaaa, go raffinr