- Actor Allusion:
- Neil Patrick Harris and Nathan Fillion play characters whose names begin with "Dr." and "Captain", respectively. Now, why does that sound familiar...?
- Continuing the Doogie Howser, M.D. allusions, the supplementary material even mentions that Billy was a child prodigy, skipping grades and excelling in (what else?) science.
- The Cameo:
- Drew Goddard at the end of Dr. Horrible's Sing-Along Blog as Bad Horse member "Fake Thomas Jefferson". He's written for both Buffy and Angel and directed The Cabin in the Woods.
- Director Joss Whedon plays one of the karaoke cowboys during "Bad Horse".
- The newsreaders in Episode III were played by David Fury and Marti Noxon, also from the Evil League of Evil, "Professor Normal" is played by Doug Petrie. All Buffy writers also.
- In the new Homeless Shelter, the woman with a dark bob and sunglasses sitting beside Captain Hammer's three primary groupies is Sarah Michelle Gellar, the Vampire Slayer herself.
- The Cast Showoff: Neil Patrick Harris has significant musical theater experience, and it shows.
- Demand Overload: This happened when it came out.
- Development Hell: The sequel.
- Jossed (quite literally): In the commentary, the cast mentions Dr. Horrible's overlarge armchair just happened to be in the house. The entire house just happened to be there, no set dressing required.
- Name's the Same:
- Captain Hammer — not to be confused with The Backyardigans superhero of the same name.
- Johnny Snow
- Both Doctor Horrible and The Big Bang Theory include an Only One Name character named Penny that knows/is known of by a dork played by Simon Helberg and pined over by that dork's also dorky friend. The resemblance stops there.
- No Export for You: In some places outside America. Good thing the DVD isn't region-locked.
- Playing Against Type: Nathan Fillion as Captain Hammer.
- Recycled Script: Two examples from Angel:
- There was an episode about a shy scientist who liked to sing and was in love with a red-head. He built a machine that could stop time to solve his relationship problems.
- In Season 5, in which the death of a character was used to get into an evil organization. Angel pretended to be involved in his friend Fred's death and really murdered another to infiltrate the Circle of The Black Thorn and Dr. Horrible's involvement in Penny's death filled the qualification to enter The Evil League of Evil.
- Viral Marketing: The word spread about Dr. Horrible by word of mouth and word-of-internet. So fast and so far that the server was not prepared for the load and went down.
- The Wiki Rule: Dr. Horrible's Sing-Along Wiki.
Trivia / Dr. Horrible's Sing-Along Blog