Teddy didn't know what to say to Billy's protectiveness. He was touched, but he was a bit emotionally retarded when it came to talking about his feelings. So he settled for a, "Thanks man. That'd be really cool of you" and hoped that Billy understood that Teddy really did appreciate the gesture. Because if he ever did get stuck as a wolf, he didn't want anyone but Billy put in charge of him or whatever.
"No worries. I'm in enough control to keep from attacking any of your bits." He'd gnaw his own leg off before he harmed Billy. Accidents happened, but he didn't think he'd lose his control long enough to hurt his best friend.
Thinking about it for a moment, because it sounded like Billy wanted to do this Halloween thing, he finally came up with, "Superman? And Lex Luthor?" It was a thought! "I mean, we always joke about it, but why not? I'd even play Luthor cause you have that tall and dark thing going for you." Teddy could acknowledge that Billy was handsome, but never aloud.