If you haven't seen it already, take a look at this:
Harris earlier talked about the thing teased above on EW.com; link here. The "Dr. Horrible" discussion starts about 2:17 in.
By the way, if you decide to watch the entire interview, I should note that "Slappy and the Stinkers" is on DVD. And that "Forgetting Sarah Marshall" outperformed the second "Harold & Kumar," according to Box Office Mojo.