Perhaps I'm wrong here, but from my understanding, in order to do motion capture, you need sensors attached to a moving subject in order to input the motion data.  Such systems are extremely expensive.

A solution I've found in working with pre-existing bvh files is to use frequent editing to splice together motions in a way that allows you to use segments of motions that may not go together otherwise.  If you look at the average Poser animation, the action seems stilted in part because the editing does not resemble that to which most moviegoers are accumstomed.  Cuts in films come about every 5 seconds or less, so you can render scenes for a shorter period of time--just enough to get the action you want--and then piece the short bits together in an editing program.

Mike