I recently did Pajama Game, we did oversize pants, without any zipper or elastic and they would only stay on if they were held. so when he let go they dropped another thing is you could put weights in the top of the pants so they fall, but you need to worry about the sound when they dropped on stage. Looking back i wish I would've tried the weights.
it was touch and go for most shows, but the actor was really good at making them fall or acting like they did after he pulled them down, is pretty much just trial and error.