Visual effects today are unbelievably realistic -- probably because they require an equally unbelievable amount of work by a ton of people. Before said artists get to have any real fun creating terrifying prehistoric or supernatural beings, many visual effects employees start out as roto artists. Sometimes jokingly referred to as a glorified tracer, the roto artist uses computer technology to meticulously extricate specific components of many frames of animation or live action movies [source: Tech-Faq.com].
For example, let's envision a film that might require Russell Brand to be placed on Pluto (the former planet, not the dog). Brand shoots his scene and then the roto artist is handed the largely unenjoyable task of tracing the actor's likeness, down to each and every errant strand of hair (of which Brand has many). In the next frame, Brand moves ever so slightly and the artist repeats the task in painstaking detail ... over and over again until the scene is finished. This glorified cut-and-paste probably isn't all that bad for a frame or two, but one can imagine the unbelievable tedium of completing this task a hundred times.
Once that's complete and the roto artist heads out for a much-deserved happy hour, someone higher on the visual food chain gets to actually place Brand on Pluto, where he will undoubtedly wow Plutopians with his trademark British wit.