I want to make a door that can't be opened. Basically, when the player clicks the door, I want the opening animation not to play, and the NPC standing in front of the door to initiate dialog with the player saying that he's not allowed through. How would I do this?

You want to initiate Dialog? Or just use a MessagBox?

MessageBox script:

Beginif (OnActivate == 1)	Messagebox "This door will not for you."endifEnd

And to initiate dialog, just use:
"NPCs ID"-> ForceGreeting
Instead of the MessageBox line. Then create the Dialog as normal, under the Greeting tab.

I went with the message box instead of the guard. It was easier, due to the guard already having a regular greeting, and I figured if the player is sick and kills the guard they still won't be able to get past the door.