It's a testament to Seth MacFarlane's vocal talents that his voice doesn't leap out at you as much in this series (or The Cleveland Show, as MacFarlane's only character he voices in that series is Tim the Bear, Cleveland's next-door neighbor). Most viewers can figure out that Seth voices Stan (since the voice of Stan has been used on Family Guy in a couple of parodies of 1950s educational short films, like the one on "I Am Peter, Hear Me Roar" about how men should treat women in the workplace [which, since this is a 1950s film, includes a lot of emotional manipulation and sexual harassment]), but a surprisingly large number of people aren't aware he voices Roger the aliennote though, if you watch both this show and Family Guy long enough, you can sort of tell that Seth voices Roger because Roger's crying sounds exactly like Peter Griffin's. On top of that, Seth actually broke out his Roger voice during his monologue on Saturday Night Live and a DVD-exclusive special on how American Dad was created and the table read for the second episode "Threat Levels" and Greg the newscaster.
Hayley is voiced by Rachael MacFarlane, Seth's younger sister, which leads to a lot of incestuous undertones as both Seth and Rachel have played characters who get romantically involved (Stewie Griffin and Olivia; a couple episodes of American Dad have had references to Hayley being romantically involved with her father — in "Francine's Flashback," Stan's plan of revenge after Hayley's boyfriend runs off with Francine is for Stan and Hayley to date each other [until Stan realizes that he didn't think the plan through], and "Pulling Double Booty" was all about Hayley dating a man who actually looks like Stan).
Missing Episode: The Christmas episode "Minstrel Krampus" that was scheduled to air on December 16th, 2012 was pulled following the Sandy Hook Elementary School shooting. Unlike the Family Guy episode "Jesus, Mary, and Joseph" (which was also pulled, yet that episode was barred because it only featured threats against killing a child [in this case, Jesus] and some people would have found the religious jokes to be in poor taste), it's currently not known when this episode will air (as "Minstrel Krampus" does have scenes of gun violence that would be considered in poor taste because of the shooting and features a demon that kidnaps naughty children), as the episode that aired in place of it on December 23, 2012 was "National Treasure 4: Baby Franny: She's Doing Well: The Whole Story." It's now scheduled to air on December 15, 2013 as the ass-kicking Christmas special we never got to see in season eight.
What Could Have Been: Originally, Laura Prepon (Donna from That '70s Show) was supposed to voice Hayley Smith, but was replaced by Seth MacFarlane's sister, Rachel. Had Prepon stayed, the show would have gotten a lot more unfavorable comparisons to Family Guy as the latter show also has someone from That '70s Show as a cast member (Mila Kunis replaced an uncredited Lacey Chabert on Family Guy and played Jackie on That '70s Show).
Klaus was originally French but Dee Bradley Baker didn't know French well, so they retooled the character to make him German.