It probably all boils down to terminology and how to interprete it, but to me 'acts' bring foward a vision of being restricted in your travels. First you have to kill some bad guy and then the door opens to the next act. This does not mean you can't go back after that. But first you have to kill that one bad guy (or girl <img src="/ubbthreads/images/graemlins/winkwink.gif" alt="" />).
I don't know how it will be implemented in Riftrunner of course.
