In paragraph nine, what does Miss Brill realize when she sees the little dog?

From the text:

But it wasn't till a little brown dog trotted on solemn and then slowly trotted off, like a little "theatre" dog, a little dog that had been drugged, that Miss Brill discovered what it was that made it so exciting. They were all on the stage. They weren't only the audience, not only looking on; they were acting. Even she had a part and came every Sunday.