3D modelling, rigging and animation, compositing, layering, matte painting, motion capture, motion tracking, color grading, and so forth and so on. Such is part of life of the visual effects department--making the unreal seem real on the movie screen. Below is a collection of before-and-after reels--you can just imagine the kind of headaches people had to go through to fool the eye. My favorite would have to be Jurassic Park, considering that film was released a whopping 20 years ago. The quality of its special effects is still pretty solid by today's standards.
The Great Gatsby
The Great Gatsby VFX from Chris Godfrey on Vimeo.
Fast and the Furious 4
Harry Potter and the Deathly Hollows Part 1
Marvel's The Avengers