First off I would have to say that you do have a good eye for detail and that you show a lot of promise for your rendering skills. With this piece though the biggest thing that screams out to me is that you have some proportion problems especially with the ears as they look way to small in relation to the rest of the face I seem to remember Clint have bigger eats than that. Secondly I see a lot of facial alignment problems and the piece is coming off very skewed, the center of the eyes should line up with the corner of the mouth. Now because of this misalignment with the eyes you have compensated with the rest of the facial features so that it all leans our right.
Imagination is not a total internal power but rather it is a reflection and multi-faceted projection of our experiences and knowledge. We take in information from the world around us and intuitively re-order it into something new. Something is not created from nothing but simply transformed from what was before.
i second ITo's crit....and suggest you overlay some tracing paper and draw the skull as it should appear at that subtle angle.
(scored to an IVAN MAXIMOV Classic animated short )