rotobrush 3 - keep viewport updated when generating mask

You can step through each frame as it renders by pressing Ctrl/Cmnd + Right Arrow. I'll do this on complex sections.

After trimming the shot, color correcting to improve edge detail, sharpness, and contrast, then Pre-composing with Trim and Move All attributes selected, I usually draw the first paths and clean up on the first frame. I then apply the Refine Edge tool, move the CTI (current time indicator) to the last frame of the layer, and let Rotobrush propagate. 

The next step is to go back to the first frame, then step through the timeline slowly and look for any problems. I often do this with the different view options selected and, if needed, step through the shot one frame at a time using the keyboard shortcuts.

When all the corrections have been made, and I am on the last frame, I will freeze Rotobrush, then pre-compose again, naming the comp Roto Matte, then Pre-Render the Rotobatte using the composition menu. It usually only takes a couple of seconds. 

If properly shot with roto in mind, adequately color-corrected to enhance the edges and contrast, and then pre-composed, the latest version of Rotobrush is incredibly accurate and quick and usually does not require many corrections. Just three years ago, it took me more than three days to adequately rotoscope a seven-second shot and complete the composite. I could probably do the same shot today in a half a day.