Hasta La Motion!

So I recently noticed that a lot of TV programs have actors with amputated limbs (i.e. Michael Shanks from Stargate Continuum, Genevieve Cortese from the latest Flash Forward, Suleka Mathew from Hawthorne) and I was wondering what was the best method of creating these digital amputations?

I was thinking of rotoing the leg out and having a clean plate in the back to fill in the gap but unless it’s a locked off shot, it seems the process would be time-consuming. I was also thinking that the limb is roto-ed out and then the background is painted back in.

Does anyone know of possible methods to recreate these digital amputations?