To rotoscope in photoshop, should I shoot the subject twice, once with the subject, once w/o? Would it work if I wanted to take of a body part, and the background will still be there? please help

I really don't know what rotoscoping is, but I think I know what you are talking about.

Yea, film the subject an then the background at seperate times. Keep precise measurements and you can make it look like teh dude is still their and you can make it look like his arms are flying off.
The reason you want two, is if you just record the dude, and you take out his arm, you have the blank space behind him and have nothing to fill it with.
hope that helps.

the answer is yes.

Jesus. At 30 frames a second that has got to be a pain to do.