Cassaday was invited to direct 'The Attic' by the show's creator Joss Whedon, with whom he worked on Marvel comics Astonishing X-Men.
"It was about the span of a year between him asking me to do it and actually getting the green light," Cassaday told CBR.
"The big stumbling block was [the series] just getting renewed. Once that happened, it was up to Joss to work his magic and get me through the right doors. I truly didn't get too worked up over it until I was completely approved and given shooting dates.
"Every day was a learning curve. There was no rhyme or rhythm to the shoot. One day would be the best - I'd be on top of the world and felt in control and great, and then the next day would be like running underwater.
"We just couldn't get ahead. Lots of tough days, but the good ones made it all worth it. And I'm happy with the footage we got every single day."
Cassaday added of 'The Attic', which premiered on FOX last Friday: "Playing throughout the episode is a subplot that turns out to be a game changer. By the end of the episode, the entire series will take a terrific turn."