Achieving a "realistic" charcoal effect (on people portraits) using native Photoshop functionality is very difficult unless you have traditional artistic skills (I don’t).
Kicking and screaming, I’ve recently gave in and acquired Corel Painter 8. Like Photoshop, it’s not intuitive to learn by any stretch. Like Photoshop, when you get used to its quirks and get your arms around its capablities and some techniques to apply them, the results can be very impressive.
Alternative II: If you can get your hands on a copy of Microsoft Front Page 98 (through eBay or the like), it includes a Photoshop compatible plugin called "Impressionist." This plugin generates some extremely realistic looking chalk-like, charcoal and pencil stroke effects. There’s nothing on the commercial market like it.
For a plugin, it does a pretty decent job rendering charcoal effects.
~DannyR~