This is a pet peeve of mine, too. FYI... professional FX houses use a technique involving a special secondary render pass that uses "one-bounce to camera only" ray tracing count called an Occlusion Pass (usually w/ Renderman or Mental Ray) to determine if areas of a surface are concave (like mouths, nostrils, armpits, and other body skin-fold crevices) and use this extra render pass in a clever composite to darken those areas, since they would by nature recieve less light than their surrounding regions. I've tried with no success to duplicate this occlusion pass effect with firefly. Since others have dealt fairly well with the nostrils already (look in freestuff) I've been toying (slowly and intermittently) with the workaround idea of creating a "dark translucent morphing mouth ball prop" that will partially fill the open mouth and darken it appropriately -- it's a little tricky, but if I meet with any success I will share it with the community. Anyone else's input and sharing would be great, please let me know!!