Solve : adobe after effects. transparency for video?

1.	Solve : adobe after effects. transparency for video?
Answer» I have a video. I want to keep one guy WHOS talking. And the rest background, i wish to make it to green/white/ transparent. Im having problems with roto brush. Also I want to apply that modification for ENTIRE video, but this happens only for 1 second. Someone COULD explain me, or create my video WITHOUT background. More details: https://www.youtube.com/watch?v=te6oUMhay34

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