The actor played eccentric business guru Robert California in the NBC comedy's seventh season finale and will reprise his role in future episodes.
"I never really considered whether I would do more [episodes]," Spader told The TV Addict. "Then....suddenly [the producers] called me back again and said, 'We'd really... like your character to come back in some capacity'."
Spader explained that California is "not as concerned about being loved" as Steve Carell's absent Office character Michael Scott.
"He's very different and brings with him a very different energy," explained the star. "We just started shooting and right now he's sort of doing what I'm doing, getting to know the office, all the people and trying to figure out how to make it all work."
Spader previously described his character as "strange" and "enigmatic", while Steve Carell praised the decision to hire the actor, "calling him "an excellent choice".
The Office returns to NBC on Thursday, September 22.
Watch a new trailer for season eight of The Office below: