Writing a movie sequel can be both easier than an original story, because the characters have already been established, and harder, due to the constraints placed on the characters by the original story. The studios commission most sequels, but a script sequel can be used as a calling card of your talent for.
You can find the script to nearly every movie on the Internet Movie Script Database (IMSDB). Take note of how writers succinctly describe characters and settings in just a few short lines! Reading scripts is a good way to learn about dramatic plot, too.