ActionScript 2.0 :: Line Of Site, Raycasting And Realistic Visability?

Ever notice how if your sitting in a room, and theres a wall in front of you, you cant see whats on the other side of that wall (and if you can, you should be here...) I'm working on a version of icio's "Point-to-Point Line-of-Site" Thingy.

Think: Top down guy walking around in a building, with multiple rooms. He cant see whats in the room (since like I said, the walls are in the way), though, he can see into the room through the open dooor, but only a small part of it (since the door is only a thin opening in what is an otherwise large room.)

What I want to do is make realistic, dynamic lighting, based on (I would assume) raycasting or lone of sight or some similar godlike power.

I edited Icio's demo with MSPaint showing what I am talking about. I've been thinking about ways to do it and have thought of a few different ways that would be, for the most part, processor intensive by searching around for visable points based on our hero's position and what he can "see" Any one have any ideas for an easy(for the processing computers atleast) way to do this?