Okay, Terrence Mann, "American actor, director, singer, songwriter and dancer who has been prominent on the Broadway stage for the past three decades." (And of whom I haven't heard. There's prominence for you!)
In retrospect, I assume he's talking about acting rather than writing - but, I don't know, maybe the same rules might apply?
In my multi-faceted writing life, I haven't ever written anything specifically for the stage. I HAVE however, transcribed a screenplay into a stage play, which got to the finals of a contest a couple of years ago - and am in the process of doing that again, for this:
"Equal Writes calls to all playwrights to submit short scenes and
monologues of 3 to 10 minutes long with the focus on women, women’s
stories or women in situations we are not presently seeing represented
on UK stages."
It's not so easy to do. Of course, dialogue is fine, but all those fancy-pantsy little ideas and locations I can fling in at random in a screenplay and leave it up to the director to magic out of a cinematographic hat - well, they REALLY need careful thought if I'm to achieve the same effect on stage.
Theatre may not be making me good, but I tell you what, it's certainly making me THINK!