1. Everyone Forgot About the First Read-Through
Well, not everyone -- Felicia Day showed up early, and she brought pastries. But all of the male cast members -- Harris, Fillion and Simon Helberg ("Moist") -- forgot about the reading and had to be called. Fillion and Hedberg immediately raced over, but Harris didn't get the message until hours later because he was taping. So for the first read-through, Dr. Horrible's part was read by Joss Whedon's brother, Jed. Harris showed up for the next one, but Jed wasn't cut entirely: he was one-third of the Bad Horse chorus, and he played Dead Bowie.

2. Ben Edlund Was the Superhero Punster
Before he became producer on Angel, Firefly and Supernatural, Ben Edlund was a comic book creator, writing and drawing the superhero parody book The Tick and overseeing the animated and live-action adaptations. When Whedon developed the Horrible concept on the picket lines of the 2007-2008 writer's strike, Edlund was there, and came to a few planning meetings before dropping out of the project. But it was his bottomless supply of supervillain jokes that gave the show Horrible's damp roommate Moist and the equine crime lord Bad Horse. Fun fact: Edlund also once created a character called the Man-Eating Cow. Coincidence?

3. Nathan Fillion's New iPhone Was Invaluable
Not only did Fillion choose to read his lines off of his iPhone at the first read-through, he also used it to take the head shot that was used for the Captain Hammer fan T-shirts. And Doc Horrible's iPhone program to drive the van by remote-control was designed by a friend of Fillion's, P.J. Haarsma, who keeps a copy of it live at Softwire.com. [Note: Does not control vans.]

4. You've Seen Dr. Horrible's Coat Before
Originally, Whedon had envisioned Harris wearing a regular doctor's coat, like a grown-up Doogie Howser M.D., but costume designer Shawna Trpcic wanted to avoid that, so she came back with the mock-turtleneck coat he eventually wore. And since Fox was loaning them most of the costumes for free, the one Harris ended up wearing was actually the same coat worn by Simon Tam in Whedon's Firefly, as well as the same coat worn by the scientists in Alien Resurrection -- another Whedon project. It's like another cast member, at this point.

5. Penny and Moist Supplied Their Own Costumes
The majority of Felicia Day's outfits came from Fox, since they needed duplicate copies for stunts, but Trpcic also raided her closet to fill in the gaps. Ditto Simon Helberg, who had plenty of T-shirts to go under Fox-supplied shiny jackets. And while Billy/Dr. Horrible's nondescript street clothes were Fox-supplied, the look of the plain hoodie-T-shirt-pants combo was based off of a photo of Jed Whedon, taken at a meeting after Joss pointed out his look. "You know, boring! Like my brother!"

6. Everyone at Harris's Gym Thinks He's Crazy
Because he had to match his mouth movements to the prerecorded songs, Harris had to practice lip-syncing all of his musical scenes. He did that by putting one song on a loop on his headphones while running on a treadmill at the gym, often for hours at a time. So anyone looking over at Harris from the Nautilus machine would have seen him soundlessly emoting to his own soundtrack. Please, please, please let there be security camera footage.

7. Bad Horse: Samurai?
Apparently, no one felt it necessary to tell the costume designer that Bad Horse was actually... a horse. Seeing the name on her list of villains and thinking of the Four Horsemen of the Apocalypse, Trpcic designed a full samurai costume for a human crime boss, complete with a horse-tail braid and saddlebag-esque pants. She was eventually told that Bad Horse would be a plain old horse, and she felt embarrassed. Maybe that can be part of Bad Horse's origin story? A samurai cursed to live as a horse? Whedon, we smell prequel.

Order your copy of Dr. Horrible's Sing-Along Blog: The Book here, and see our list of Joss Whedon's greatest episodes.

What are people saying about your favorite shows and stars right now? Find out with Talk Without Pity, the social media site for real TV fans. See Tweets and Facebook comments in real time and add your own -- all without leaving TWoP. Join the conversation now!