Perhaps I plan my own faces too much, and it hinders their expressiveness.

Yours are good! I really like the expressions.

Thank you Merrak. I find it easier using loose strokes when I'm drawing expressions. Too much concentration takes away from feeling. A lot of artists practice gesture drawing where the key is to get down a body gesture quickly without too much emphasis on form. These drawings usually look scribbly but they convey feeling very well. You might even be able to work a gesture up into a finished character.

When I try to draw different expressions, I don't spend a whole lot of time on them, I just add the main details of the character so we're sure who it is!
Maybe trying to make an expression sheet!
It doesn't have to be made on the computer, label it on top - with the Character's Name, and make like a grid and focus on the face, not the hair, just the face and work on expressions!

Something like this:
http://napalmnacey.deviantart.com/art/2 ... s-55523083
http://2.bp.blogspot.com/_oR7sYJzWZ1k/T ... al_02a.jpg

After when making an actual drawing with flesh tones and stuff like that you can add more detail with shadows and the hair and surroundings to make it more dynamic.