Wait, The Show Started and I Haven't Even Taken My Seat!!

I enter the Acorn theater. The usher guides me to my seat. As I walk to my aisle, I look up and what do I see - Marie and Bruce in the middle of the stage in bed.  

As I finally make it to down the left side of the theater and slip into the first row, Marie casually gets up and walks off stage into the next room - a bathroom break perhaps in the middle of the night. Bruce is still asleep. Marie makes her way back to bed. 

The audience continues to spill into the theater. Marie lights up a cigarette. I am happy that this is theater because dosing off in bed with a lit cigarette is probably not a safe idea. I look around the theater, and there is the playwright Wallace Shawn two rows back (I'll never forget that face from The Princess Bride).

Then I sit back and wait for the dialogue to begin. And the show to start? Or has it already begun?

Examples of shows where the action starts before the audience is fully seated.