UI ListBox on RollOver

Any ideas on how to execute a function specific to the contents of an item in a ListBox on RollOver while maintining the Event Handler click functionality.

Basically, I want to call a function when a user rolls over a ListBox item (eg displays the contents of the rolled-over item in a field) and call a function when a user clicks on an item as well.

Seems easy. What am I missing? The answer is worth one American dollar.