How to activate an area during gameplay?

[FlawlessHappiness]

You can make the player incabable of controls with the function
SetPlayerActive(false);

Then with different script lines you can make the scene you want.

Use this:
http://wiki.frictionalgames.com/hpl2/amn..._functions

and Ctrl+F to search

MovePlayerForward has to be repeadetly called by a timer, but is a function that makes the player walk.