Most experienced public speakers know how to deviate and alter and add flourishes to their prepared remarks on the fly, but few do it as well as Clinton. (Even if you disagree with what he's saying.) As you can see below, from a purely rhetorical standpoint nearly all of his changes enhanced the text in some way and brought added emphasis to arguments. Notice his frequent changing of "should vote for Barack Obama" to "must vote." And his even more frequent use of "Now" and "Look" when beginning a point. Many of his best lines — like his "bloodsport" quote — were either ad-libbed or added in back in at the last moment.
The Bill Clinton speech that Louis CK calls "one of the greatest things I ever saw"
How Bill Clinton handled a heckler back in the day