This was taken using a Pentax 645 with 55mm lens using the "in your face" method. If you want to take candid photos with a medium format SLR, there really isn't any other method. I do usually prefocus the camera (or at least set it close) so I don't have to spend too much time with the camera to my eye. I can just pull it up and fire.
I use a similar approach when I'm out with my little Retina IIIc. I usually prefocus, but people really don't pay that much attention to that camera. I guess because it's so small and quiet.