A demo reel is a montage of your best acting work. It might also be referred to as a showreel or sizzle reel.
The important thing to focus on here is the word “best.” Casting Directors and anyone else looking at your demo reel won’t be all too concerned with whether the footage in it exactly matches the role that they’re casting for. They just want to make sure you have the talent, screen presence, and other relevant acting qualities to do the job.
It’s also important to remember that people–including Casting Directors– have short attention spans. You need to show them not only your best acting work, but also your best acting work first. Don’t save your most impressive bit of acting for a big demo reel finale because it’s entirely possible that a Casting Director or Director won’t make it that far.