For me it's a combo of anatomy, shading and perspective. I'm still working on them. I wold have love to take a class on the subject, but the corses are freaken expencive here in my country. =_=; The rest comes easy for me. I have discovered that I have a undying love for spawning new characters. Sadly I don't do much with them even thopugh that I have a separet story for each and every one of them.