The wisp moves like a brained roach, more or less (there are subtle differences but they're hard to explain without giving specific examples). It always tries to move one square closer to Beethro.
If Beethro steps on the wisp then everything beyond him vanished from the wisp. If Beethro just steps adjacent to the wisp, the wisp will continue as normal. But when the slayer starts moving, he will always choose the shortest path to Beethro. So in the following case:
B
xxxxx
xxxxxxxxxxs
Where Beethro doubled up and caused a two-square wisp, the slayer won't follow the original path of the wisp but will turn north early.
As for speech - it's possible to make Beethro, the Slayer, and Halph talk without them being characters. It's not possible to do so to arbitrary monsters (it involves using an invisible character to hold the script and a special hard-coded way to associate the lines with the right person).
____________________________
I got my avatar back! Yay!