Undead scientist
Under the dome building where you go down a manhole to the sewer (because the entrance was blown up), is a man in the corner who is not dead. Shoot him twice when you are talking, and he should stand up and run. That is all he can do.

Running from the monster alien
When you are right at the end of the demo, get to the right side of the platform and keep running against the edge. When the monster approaches to you, he will hit a computer and it causes the platform fall. Keep running to get past all the debris. You will be able to run around for a short time before the demo ends.