Watch a video of South Park in the scrapbook after unlocking it in a level. To unlock videos, you can:
Kill a new type of enemy.
Use a characters special ability.
Get gold or higher medals on levels.
You can also get this by pressing Back as soon as you unlock the video in game. A little icon will appear in the top right corner saying when you have unlocked one, and when you see this, press Back to watch the video. Any video will do.