Then you begin to forget - first its the lines of dialogue that go, lines that you knew backwards and could recite ahead of the actors. You know the scene in which Myra confronts Eliot on his return and certain phrases pop out: "He's said nothing about you since you left" and "But I haven't hurt you" - but the order of the lines is slipping. Then the lines themselves go, then the order of scenes in the play itself, until finally it's a blur and all you remember is what you were feeling when you watched it.
Even then, what you were feeling as you watched it is an unreliable test, because as the writer you feel a raft of emotions completely contradictory to those that you want the audience to feel. You're worried about whether the actors will remember their lines. You're worried about why the couple two rows in front of you are whispering to each other and staring at their phones. You're worried about whether you'll ever get paid to do this, because apparently you love writing so much. If you've written it well, you will also care - as you hope the audience does - about the characters on stage, at least as much as you can whilst holding all those anxious thoughts in your head. The worst thing is, anxiety is pointless. When everything goes right, your anxiety is proven to be a waste, and when something goes wrong, it's completely out of your control anyway.
In fact, it's when things go wrong that you can have the most fun. On one particular night of How We Lost the Son, a certain actor forgot to bring a certain important prop on stage. He was to present this prop to another character, who - knowing the deep significance of it - was then to react violently towards him, in a song that made great use of this prop in its blocking. Watching the actor sing that song without the prop he needed was amazing. He improvised. He tore at his hair, he clenched his teeth, he jumped up and down, he went off, he rocked out. He conveyed exactly the emotion I wanted that song to convey. He conveyed exactly the emotion I first experienced when writing that song.
The reason I write is because I find many things moving and interesting, and I think other people might find these things moving and interesting too. I also write because it's fun. But half of the fun of watching a story unfold is the discovery of the story - the development of a plot full of unexpected twists and turns, the sudden moments of pathos found in characters whom you have come to believe as real, the payoff of a satisfying conclusion that answers all your questions, and the compression of all of these discoveries into two hours. When you write, the process takes months, not hours, and the development of those unexpected twists and turns, which seem like sheer inspiration to the audience, are the result of hours and hours of writing, cutting and rewriting. The moments of pathos are half-formed and only grow to their full when an actor (a good actor) takes your words and breathes life into them on stage or in front of a camera. The satisfying conclusion will still, hopefully, be satisfying for the writer, but you'll have exhausted yourself getting there. Writing is like exploring - hacking a path through dense undergrowth in a dark jungle, being the first to find a path from one point to another, and then leading your audience in safety along that path. It's fun, but it's a different kind of fun to that which the audience enjoys.
This might seem melodramatic, or archaically Victorian (conquering the dark jungles and all of that) but I hope it's a useful insight into the experience of writing. No one really knows how anyone writes anything, where inspiration comes from or what is needed to form inspiration into entertainment (and there is a massive gap between the two, which I might go into at a later date). All we can do is keep working.
