For as long as there have been movies, there have been special effects.

One of the earliest film effects is "compositing," which was famously used by Georges Méliès in his 1898 work, Four Heads are better than one (Un Homme De Tête). Compositing is the technique of blending multiple images into one to create illusions, like four of the same heads being on screen at once. 

As movies evolved over time, filmmakers used other means to create illusions, like the "back black matte" technique, where actors were filmed in front of a black background; a blue screen three strip technicolor process when films moved from black and white to color; a "yellow" screen technique that was used in the original The Parent Trap; and in addition to this, the use of paintings as backdrops in scenes that called for something grand, like fantastical landscapes or a large crowd, such as Chris Evans' work in the original Star Wars. 

Then came along the green screen, also known as Chroma Key, which took over as things moved from film to digital. Today, the magic of movies is largely regulated to computer generated special effects that use filming in front of a green screen to separate the actors from what will be added in later. But use of the green screen can make or break a movie, especially through overuse. George Lucas relied heavily on it when he developed the Star Wars prequels, and was criticized for "ruining" the franchise with it, among other things.

Using Chroma Key techniques isn't easy, even if they're done with computers. They are massive undertakings that take a ton of money and a lot manpower to pull off. And as our hunger for bigger explosions, larger monsters, and crazier action scenes intensifies, visual effects artists, engineers, and scientists have to keep upping the ante to satisfy our expectations. Yeah, using a green screen to switch out backdrops for your YouTube video is easy—but building a working world out of it is an entirely different thing.

Still, it doesn't mean filming in front of a green screen can't be awkward. "It was so distressing and off-putting and difficult that I thought 'I don't want to make this film if this is what I'm going to have to do'," Ian McKellen said after filming the The Hobbit. "It's not what I do for a living. I act with other people, I don't act on my own."

"I felt pretty miserable… and thought perhaps, has the time come for me to stop acting altogether if I can't cope with these difficulties?" he continued. That's right, the green screen actually got Sir Ian McKellen to consider quitting acting. That's no small feat. Why didn't he feel the same way during filming The Lord of the Rings? Well, Peter Jackson was praised for using techniques like forced perspective to that kept the environments in the boundaries of reality—as much as films like that could. The Hobbit, on the other hand, used CGI much more, and Jackson was dissed for it just as much as Lucas. 

Here are scenes from some famous movies and television shows that show the use of green screen, and the worlds and characters that can be created with them—for better or worse.