Damn! I find hands harder than faces. There are so many little muscles and bones in them...and hands can convey almost as much emotion as a face, too. When you're using such a limited color and line style, each littl mark you make counts. It's more challenging in many ways than what I do when I paint.
So - the thickness and thiness variation of each line helps describe the form, like in comic art. And placement of each line is critical! Also design composition - the shapes and placement of the figures. Composition can make or break an artwork, take it from ok to wow. In the case of Dexter, think about where the light direction is coming from... maybe somehow darken either him or the newspaper, to creat a depth of field? Since his eyes tell so much about what he's thinking, maybe think of a way to emphasize that?
If I were painting him in color, I might first simulate the lighting effect I want in Photoshop, then use that as a guide when I paint. It the way I usually compose my artwork before starting. of course I'd use value and color to accent the things I want, I usually deviate from the ref photos, add or remove things, etc. But I have alot of "wiggle room" in case I make mistakes, I just go back in and re-work them til I get it right
In your work, it's hard...I give you much credit there! You have to get it right the first time! Maybe study some other great pen and ink artists, comic artists that do linework, see how they tackle those same issues. Have you tried other marker types? Like brush markers, or rapidographs? I used to do stipple pen and ink, which is a bit similar to your style (using all dots to create the image on bristol board) I used a rapidograph, but they're a pain to keep clean, the nibs are expensive! It was really precise, though. Brush markers are cheap, have a nice thick to thin stroke...I use them now, when I'm drawing tattoos in my paintings.
I hope this helps a little...but you're doing great with your style just as it is! I know, we artists always want to take it to the next level