I've certainly visited enough old graveyards to have a pre-existing attraction to old crypts. And, having visited a number of old European cathedrals and even the Cloisters Museum in my neighborhood, I've seen enough old stone coffins. I've always loved how thick they are, how nicely carved. Those were my starting points.
Innistrad had a couple of art direction rules, one of which was that there were to be no "helicopter shots." Meaning, shots from so high up that it detaches the viewer from what's happening. All camera angles were to be composed so that the viewer is himself in the world. A subtle thing, perhaps. One of my thumbnails had a top-view (left), in which you would be standing above the coffin as its lid is being slid away. I kinda liked it, though I would have eventually changed the arms to either both be over the lip of the coffin, or both pushing the lid, as it's awkward like this. Not impossible--I pantomimed it first, but just awkward. Technically this isn't a helicopter-shot, but it wasn't worth me sketching it out to be rejected, given it was on the line, so I left it alone. I did return to a part of it, which I'll get to in the next Innistrad post.
Most images still start with some kind of photo-reference, in which I or others I can rope in get to pose and look ridiculous. I'm just vain enough to not show them here, but others love this bit of cheese. The incredible comic artist Paolo Rivera, for instance, has a weekly installment on his blog, over 100 posts long already, showing off just this sort of thing (here's a link to just that series, but see the rest of his blog, too).
Using the self-timer on my camera, I played with a few hand poses based on the thumbnail I eventually chose. Good times. Much as you can eventually learn to fake lighting, there's nothing like the real thing to show you contours and shadows you would never have thought up on your own. With the real thing in place, you can often fake extra bits and have a nice convincing mix. Once I choose my reference photo, I always knock it to grayscale. After all, I don't use colored gels or anything when I take photographs, and don't use costuming I actually intend to use in the final, so there is little need for color reference. Additionally, having it will often subconsciously make me want to use it in the painting. For something like a traditional portrait or landscape, it might be helpful to have it, but for fantasy illustration, I just drop it entirely.
"Ghoulcaller's Chant" study, purchase information here
I needed to make my own healthy hands into the dessicated flesh of the dead. A little anatomy was helpful here, and a sort of imagining what the sinews would look like minus the fat and fluid, for instance around the tendons on the thumb, and so on. A ring worn that once fit is now totally loose. From there, off to paint.
"Ghoulcaller's Chant" final painting, purchase information here