Dialogue is employed to advance the actual plot only when it is impossible or impracticable to do it with dumb show.

And when I think of some of the lines I've been called upon to spout, I can't say that I regret the movies' lack of dialogue.
"What does hurt, though," he admitted, "is the absence of response.

I don't mean applause, but the something that comes up over the footlights to you from the audience, the big something that tells you instantly whether you have hit it or missed, whether you are ringing true or false.

You don't get that in the pictures.

Your audience is the director, and you know that it will be weeks or months before your work is going to get its test.
"But in everything else, the movie has the talkie skinned a mile.
Instead of mouthing somebody else's words, you are doing the thing yourself.