"On the set, Peacock shot thousands of high resolution photographs of recreated scenes, visual elements and a stand-in actor depicting McVeigh in action. Mechanism then built a CG 3D model of Timothy McVeigh’s face and replaced every frame of the actor’s face with it.
Mechanism’s unique approach to facial replacement had never been used in documentary filmmaking before which gave the compositor every possible frame of final rendered CGI in advance. In developing and refining this technique, multiple 3D models of McVeigh’s head were pre-rendered every two degrees of rotation on two axes. Two sequences of frames were a combination of 11 expressions and 11 eye positions. The number of images was then doubled by rendering two camera focal lengths to match the lenses used on set; totaling over half a million images and layers. These images were brought into After Effects with Mechanism’s custom expressions allowing the compositing artists to rotate McVeigh’s photo realistic head in real time within the program."
See also Todd Kopriva's Face replacement with mocha from David Torno on tutorials on how Torno did tracking, compositing, and other post-production work for a music video. The cmiVFX training Nuke Facial Replacement Techniques looks interesting too.
Here's a little "making of" for the McVeigh piece (the embed code is buggy, but there's a fullscreen option):
Update: Put Your Face Onto An Inanimate Object at AEtuts by Alexander Dohr shows simple "face replacement" done quickly.
And for an example of a different kind, check out this Spanish man, “Oscar,” the recipient of the first 100 percent face transplant. Previous transplants in France and the United States were only partial, but on the increase due to war wounds.
"Lola are arguably the world's leaders in human face and body manipulation. In The Social Network Lola completed a hundred or so shots, but a key 20 of those involved delicate face replacement to allow one actor to play two roles - the Winklevoss twins. We spoke in-depth with Lola's VFX supervisor Edson Williams about the technical process."