For me these two points are essential in differentiating plays from what I'll call routines. Let me say that I in no way mean to assign a greater aesthetic merit to one or the other I'm just trying to work out my task because recently I've been charged to create plays (as in the past I've made various routines: choral concerts, vaudevilles, magic shows, stand-up comedy, etc.).
Consider a magic show. How is magic show like the thing we tend to call a play? There's a character (the magician's persona) even multiple characters (assistants etc.); it's onstage; there's an audience. Some superficial differences are easily discarded: a lack of a coherent narrative is trivial since it either does have one implicitly (I will show you a bunch of tricks), or it doesn't but it has local punctuated narratives (I will guess your card even though I haven't seen it, I will saw this woman in half but she will not die, etc.) like an evening of short plays (The Shipment for example), or narrative doesn't matter (Waiting for Godot, perhaps); it lacks an antagonist (but what about the audience in a Harry Anderson show?); it lacks emotional stakes (but every trick might go wrong!).
And yet I think it's true that a magic show is importantly, intuitively distinct from a play for two reasons. The first is that it lacks a plays pretension of singularity. A magician will do his routine 300 nights a year. We see a routine for example rather than simply seeing it. In this way a routine's audience is a witness where as a play's audience is a participant. I know this might seem insane or naive, inasmuch as a play is performed more than once, but I feel that this is an economic rather than a generic feature of the form, and that we are encouraged to believe the play is only happening this once, unlike a routine. (The rock band I was in in High School always claimed that the show we were playing was going to be our last, but this was really just shtick and we only did do two shows anyway.) So, to be a play I think we need to understand why tonight is unlike any other night. If, for example, in the course of the magician's routine a kidnapper came onstage and tied him up and the magician had to get out of those ropes this would inch it over into play territory, whereas if he invites an audience member up to tie him up it's a better trick (we know there's no funny business in the tying) but the night stays the same.
Another way that occurs to me to turn a routine into a play is to involve the audience in concrete emotional stakes for the curator. Is the magician showing us tricks as a profession or is there something special about the telling that matters to him (and so to us) tonight? For instance the mediator of his divorce is onstage or in the audience tonight (unlike other nights) to determine from the routine if he is fit to get custody of his children. Now, since we are invited into these emotional stakes, unlike the any-old-how stakes of being a good magician, we leave audience time and enter "performance time" which seems to me an important aspect of plays.
I feel like summing up to make sure I get it straight. Unlike routines, plays claim singularity of the experience in time and establish a performance time distinct from their duration, and perhaps most importantly are responsible to answer why the activity onstage matters for at least one of its participants.