actually, it's not like that at all, there are no lines, you are given the story line and you have to improv 90% of the material including improv the fight aswell.
No... the fight is highly choreographed and they practice it for the full week or more before the "fight." There's also a tight script (pre, post, and during fight).
My favorite Ken Shamrock fight to date is him vs Shane McMahon's sister. She forgot her script and Ken had to say under his breath, "you're supposed to slap me!" She abruptly fell back into character and slapped Ken.
My dad was the main broadcasting manager for the WWF PPV shows and met Vince McMahon almost daily. All the fights are 100% scripted, 100% choreographed, and timed very precisely.
Cabby was right, it's essentially a play.