As someone who audited a three-credit "Action and Responsibility" class in college, my impression is that there is no more explanation to be had. There are some people who construct more elaborate theories which do have internals (cf. Robert Kane - actually, his "A Contemporary Introduction to Free Will" is short, readable, and accurate as an introduction to the classic theories and to his own), but the "naive libertarians" refuse to believe that it is any more complicated than that.

Edit: What am I doing? You're training to be a philosopher! You're the one who should be telling me!