If there's not enough light, you will have to use a larger lens opening, move closer to your subject, or use a faster film. If you are using an automatic flash unit that has no tilt or swivel capability, the only way to use it for bounce is to take it off the camera and aim the whole unit at your bounce surface. The only solution is to switch the flash unit to manual. To determine exposure for bounce flash manually, first find the total distance from the flash to the bounce surface and from the bounce surface to your subject. If the flash doesn't have a calculator dial, divide the guide number by the total distance.
Exposure also is affected by the size and color of the room.
It also creates interesting highlights and shadows on your subject. People will appear more realistic when facial texture and forms are revealed. Now you can hold the flash in one hand and the camera in the other. Don't worry if you're slightly wobbly in holding the camera. The flash will freeze camera motion.