It looks like they improvise a lot of their dialogue (and do it very well).
That rabbit is crazy; I'm Brian Waters!
Kotter: "You are lucky I'm truly not the vindictive or psycho type...I'd be careful from now on, and I'd just back the hell off if I were you....otherwise, the Mizzou "extension office" life might get exciting"
Kotter: "You're just gay. G-A-Y. Gay."