I have no feet, and no gravity do I feel. Errm, please help.
Lay off the mushrooms for a while...
:D sorry couldn't resist!
Both your questions can be answered by reading and understanding TestObjectWalking. What it does is:
1. fire a ray down (0,-1,0 direction) from the camera location
2. position the camera at the intersection point, plus (0, 10, 0) to be above the ground.
For the character/camera to "fall down", you'd just have to change that last bit to "position the camera at it's old location, but -0.1f in y direction - unless that would be on the other side of the intersection point".
I'll leave the details and making that framerate independent as an exercise for the reader :)
There is even a commented out part in TestObjectWalking that will make the camera "fly" when it ends up 5 units above the ground.
once you reach the "moving/animated random mesh <-> moving/animated random mesh with different behaviour (animated, stair, ladder, door, static, rope, evelator, spike, lava) collision"-level, you'll laugh about such simple problems.