Filmed in six days.
"Dr. Horrible's Sing-Along Blog" garnered the first-ever Emmy win for Joss Whedon, despite both his long and critically acclaimed career as a writer, director, and producer of many television shows and the fact that it did not actually air on television.
In an interview, Nathan Fillion said that when Joss Whedon called to offer him the part of Captain Hammer, Fillion saw Whedon's name on his caller ID, immediately picked up the phone, and said "Yes!" Whatever Whedon was calling to talk to him about, his answer was going to be yes.
Dr. Horrible's Stun Ray, which he later turns into a Death Ray, is the stun gun carried by Alliance troopers in Firefly: Der Aufbruch der Serenity (2002), held upside-down.
On the DVD, the ELE warning that flashes on the screen during the FBI warning reads "This video disc is designated for Evil purposes only, and any and all non-evil use will result in immediate self-hatred. Evil Rule of Evil #24B states that any and all infringements constitute an Extreme Offense with the maximum penalty of up to five nails driven into the offending skull and a $250,000 fire. DON'T BE A HERO!"