OK so i brought a new screen the other day and i have a question? is there a way to sleep the iMac display and watch a movie on the second (new) monitor? a bit like "clamshelling" a laptop with a second display?
You could try turing the brightness all the way down and making the second display the primary one by dragging the menubar in the Preferences app under Displays->Arrangement. And then dragging the movie to the other screen and full screening it.