The breakdown for demo reels is simple:
Demos Reels present your ability to act. They help you sign with an agent or manager. They are also part of the toolkit your reps use when presenting you to casting and producers.
Remember, this is not a piece of entertainment. You should be the primary focus of each demo. Their length should be between thirty to ninety seconds. The clips should be well lit and your dialogue should be easily heard and understood. Keep the clips separate and avoid long montages.
Last, post all demo reels to your website, IMDB and Actors Access. Add clips from jobs you have booked and add at least two new clips a year.