Motion-capture (BVH files) can be a tremendous time-saver for your project. Keyframing the same realistic movements by hand is a difficult and lengthy process. The only problems are finding the animations you want to use, and fine-tuning them afterward so they'll fit the scene perfectly. You can find free mo-cap files on the Internet, or buy them at places like DAZ, Credo Interactive, and the Renderosity Marketplace.