Photography at some types of private event, such as theater, dance and music performances, is banned completely for good reasons. It would simply not be fair to other members of an audience to obstruct their view with a long telephoto lens of fire off a powerful flash to destroy the carefully controlled lighting.
The only satisfactory way in which such events can therefore be photographed is by gaining permission to attend dress rehearsals. Event owners, organizers or sponsors sometimes allow a photographer access in return for copies of images that can then be used for promotion and publicity purposes. In some cases, the lighting may be set at a higher level and even the use of flash may be agreed.
The use of any images obtained in this way must of course be agreed with the organizers, promoters or sponsors.