Hi Burr, here are some general descriptions that may help:

http://en.wikipedia.org/wiki/Key_light
http://en.wikipedia.org/wiki/Fill_light
http://en.wikipedia.org/wiki/Three-point_lighting

But probably the best way to get an idea of the positions of the key and fill lights in MoI is to go to Options / View / Lighting options, and set Lighting style to "Custom key + fill levels".

That will bring up 2 sliders which you can adjust. If you set one of those levels to maximum and the other to minimum, that will help to show you the effect of those lights.

So if you set Key light level to 1 and Fill light level to 0, you'll see this:



So you can see from the location of the brightest area there that the key light is shining down diagonally, basically from behind your upper-left shoulder.

If you turn the key light all the way down and crank the fill light all the way up you will see this:



So you can see there that the brightest areas for the fill lights are in a different location - a bit towards the right. It's basically rotating the key light by 90 degrees around the z axis, and then tilting it downward a bit. The basic idea is that the fill light is shining generally into the darkest spot of the key light to help provide some illumination in those areas that the key light is not hitting.

In MoI's case the fill light is more diffused than the key light, it's actually not just one single direction but rather 10 lower intensity lights coming from some slight rotations of that primary direction. Then there's also one other lower intensity light coming from the vertical direction to give a slight back-light type effect. These 11 lights together are controlled by that one fill light slider.

Does setting the lights up with those different max/min combinations help?

- Michael