Theatergoers would probably be slower to realize that this play is not going to proceed like other plays do, though they would know the play has started, whereas the reader sees continuing stage directions.
Alone on stage, Moon follows directions that Stoppard drags out to entertain the reader; it could be summarized "Moon stares around the stage for several minutes and then reads the program cover to cover." Instead the prose is dramatic: "Silence. The room. The body. MOON. He turns over the page and reads. Pause. MOON picks up his programme. Pause ." I count seven sentences that tell us how Moon reads the program. When performed, this would make the audience impatient like Moon, and laugh at seeing him act the way they might feel. But it is also a funny imitation of a reader reading the play and waiting to find out the plot. We feel like Birdboot objecting when Moon says the play has started with a pause, "You can't start with a pause!" (1396). That's already a joke about the kind of experimental theater that Stoppard writes.
There are many "straight" stage directions not meant for laughs. These help the reader put themselves in the place of actors, directors, and crew. I can't note all of the fun effects here. But Moon and Birdboot are always talking over and over about the same themes, Moon about his jealousy of the lead reviewer and wanting to murder him, Birdboot about his affairs and his lusting after actresses. There are lots of jokes about the way the two reviewers see the same thing differently, one very intellectual and fancy and the other crude or vulgar. It's interesting that most of the dialogue directions are about Birdboot, who is always shifting his tone to keep up appearances: "conspiratorially," "suspiciously," "instantly outraged," "urbanely." His repeated loud chewing on chocolates would be funnier onstage.