I filmed an actor's reel for a buddy of mine recently. We watched several other actor's reels for inspiration. My buddy hasn't been in a lot of stuff yet, so rather than cut together his existing material into highlights, we filmed four different scenes from scratch. We tried to make each scene look different with the lighting/locations, and also stressed a different side of his character in each one (one he's a tough guy, one he's in love, one he's scared for his life, etc etc). Each scene is about a minute long. His agent was pretty happy with it so I guess our plan worked. Hope that helps.
Thanks Derrick. I really appreciate that. I find your considerations completely right. I might go with what you did for your friend; do new scenes. I was looking for something on Youtube and ended up finding the Marlon Brando screen test. Here's the link. I hope you enjoy it.