The glint in the eyes is what will breathe a little life into the expression.  Try creating a low intensity spot light without shadows and point it either to the face or one eye (perhaps the right).

Here is a link from RDNA on creating more realistic lighting and shaders by Olivieri:

http://www.runtimedna.com/mod/forum/messages.php?forum_id=95&ShowMessage=143845&Reply=339608

This might help tremedously.