Joss Whedon had a five year plan for the series and had already worked out how the characters would evolve over time but the show only ran for two seasons. With this in mind, you can see how the plots of season 2 could have been spaced out to four seasons.
The character of Dr. Saunders was originally written to be much older, but Joss Whedon rewrote the character so he could cast Amy Acker.
DOLLHOUSE was part of a FOX campaign called "Remote-Free-TV", which also included fellow-FOX-show Fringe (2008). Both shows were expected to have dramatically reduced commercial breaks and therefore more actual screen time.
In the season two episode The Attic (2009), Enver Gjokaj's identical twin brother Demir Gjokaj plays a alternate version of Enver's character who fights "himself" while trapped in a dream. This was actually the second time in his showrunning career that Joss Whedon cast the non-acting identical twin brother of one of his series regulars as a supernatural double or doppelgänger of the regular character; the first time was during "Buffy the Vampire Slayer," when Nicholas Brendon's twin, Kelly, played a part of Xander's personality that had become mystically separated from the original Xander.
Eliza Dushku and Joss Whedon came up with the premise of the show while having lunch and talking about her career options.