I do theatre, and one year I was in The King and I as one of the King's wives. ('Cause y'know, I wasn't out yet and loved theatre enough to play any manner of parts - it was real life that felt like acting, until I finally transitioned)
There's one scene where Anna shows all of the wives her locket, which has a picture of her husband Tom in it, and then she sings a song about it. We were using a blank locket in rehearsal, but on dress rehearsal night, myself and another wife put a random picture of this really old dude in the locket. On stage that night, Anna opened it and showed everyone the locket, and all of the wives on stage were trying really hard not to laugh. Looking back, it was kind of unprofessional of us, but that's why we did it on dress rehearsal night and not a performance.
In that same show, there was a scene where I had to bow to the King while wearing a hoop skirt, and I was wearing a trick skirt that was rigged to go up over my head when I bowed. The King and Anna were behind me, so they saw my underpants (the plot of the show is that the women didn't know they were supposed to wear underpants, so the King and Anna have to act like we're naked.
On dress rehearsal night I drew a huge smiley face on my underwear so that when they looked down, there'd be a huge smiley on my butt. They enjoyed it so much (and were really good and able to keep themselves from laughing) that for the rest of the run, I wrote a different message on my butt each night, like "this space for rent" or "yeah, you wish".
Some directors hate theatre pranks with a passion, but I think most of the time, they are good at keeping the cast awake when they could get stuck in a rut of performing the same thing every night. I wasn't the only one in the cast who pranked people.
