Added more geometry to block out to try and match concept picture. Eyes not deep-set enough to look properly menacing.
Went back to first attempt, then followed Andrianna's suggestion and scaled the eyes down, along with recessing them further into the head, to counter the "cuteness" that seemed to afflict the design.
Sculpted the lower jaw and lip to match the revised head geometry, then added teeth. Also refined the hinge that holds the jaw in place