As for casting Neil Patrick Harris, all it took was a phone call. "We all agreed there was really nobody else that should play Dr. Horrible. I didn't even get the sentence but before he said yes. And then I sort of got defensive (Whedon lapses into fanboy-speak): ‘No no no, it's really going to be good,' and Neil's like, ‘I said yes.' And I said, ‘No no no, I mean, but I mean the point is, is mean I mean' … I couldn't handle it."
Check out the full interview and story here.