Quote Originally Posted by David H. Bebbington
This I think is one of those rare instances where the kind of camera you use has major creative repercussions. I do portraits only occasionally, but when I do I try to use a twin-lens reflex every time. This kind of camera allows you to view the subject AT the moment of exposure, with this I always know when I have got the result I want, with any SLR this is much harder, and with an SLR without an instant-return mirror (such as the Mamiya RB/RZ67, which so many pros use for portraiture) you cannot know what the hell you've got and will feel you need to burn 100 frames of film as insurance!
Most portrait photographers that use a tripod aren't looking in the viewfinder when they trip the shutter. However the magic doesn't last more than millisecond, and you still don't know how great it was until you see it on film. The reason that movie makers and some photographer "tested" their subject/actors was to see if they had that magic. It may appear in the viewfinder and not on the film and vise versa.

Also I photograph a lot of kids. That expression can change in a heartbeat. I can have frames less than a second apart that are nuanced very differently.