The Star Trek actor played Chad Warwick in four episodes of the horror drama's first run, but will play the larger role of Dr. Thredson in the new season, subtitled Asylum.
"It's a really different story from the first season," Quinto told MTV Movies. "I'm in this whole season, whereas last year I was only in a few episodes, so I have a lot more of an arc to track."
The 35-year-old star - also known for his part on Heroes - described his American Horror Story role as "one of the favourite things I've done on television".
"It's incredibly fun," he said. "I love the ensemble of actors that are working on that show.
"It's a really special group of people so I'm really stoked to be among them. It's a fun world to play in. There's a lot to discover and create."
American Horror Story returns to FX and FXUK in October.
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Watch the latest video teaser for American Horror Story: Asylum below: