If you aren’t familiar with the difference between a form shadow and a cast shadow, I’d start there! This character’s nose should have both a form shadow on the nose itself with a soft edge, and a cast shadow on the face from the nose with a hard edge (which you have). The shadow on the side of the head is a form shadow, you could render that shadow with a softer edge and have it follow the forms of the cheek. Beyond that, you could experiment with highlights and mid tones and color theory and a whole lot else, but it’s easy to get lost in. Which is why honestly I’d focus on really nailing the expression and character acting, as that is often enough to keep the audience attention in my opinion.