I'm thinking that somebody should take these four notes and give them their own topic, lol. NOW FOR THE MOST CRITICAL REASON YOU NEED RENDEROSITY OR A RENDEROSITY-FAKER.

It was touched on above. Your renderer sees the LACK OF LIGHT as pure black. When it mixes some light with no light...the result is seen in terms of grayscale shadows. Anytime, which is way too often...anytime you see grayscale to black shadows on the skin of a character...then there's not enough light, not enough ambient light, not enough fill.

This may be corrected with global lighting or AO or IBL or rendersity solutions, or by simply adding a fill light...as they would do on stage or in film.

I don't want downplay some of the truly great pictures I have seen using AO or IBL or software renderosity calculations but simply remember, you can spend two days looking for the right IBL image, or getting the right AO settings, or calculating a truly realistic rendersity solution...or  in a couple of minutes you can add and adjust a fill light. Sometimes that's not the right solution - but most of the time it should do just fine.

I find the best thing about AO and IBL is watching artists use them for specific non-realistic effects, as in the establishment of a non-realistic rendering style. That's a whole other world - Aery Soul, for example, has a relatively complex rendering style which, like or dislike, is absolutely identifiable as their own. Using software to develop that kind of stuff, interests me much more than realism....and yet...  And yet, it's the pursuit of realism that's helped me understand what all these techniques do - and is very much like learning to draw realistically before jumping to cubism. You can make the jump without it, but doing realistic work - meeting the eye's expectations for "realism" - is always going to be the most technically difficult.

Sorry if I bored you all...