On a regular scripted TV show, that isn't The Big Bang Theory (2007) or 30 Rock (2006), featuring a well-known guest star is a big event. The networks will advertise that such and such guest is going to be on a show next week, and the guest may even be given a multiple episode arc, as with James Franco's stint on The Mindy Project (2012). On a show that's doing fairly well or even very well this happens once or twice, maybe three times a season, but on animated shows like Rick and Morty (2013) and Bob's Burgers (2011), famous guest stars seem to be exploding out of drawers, and no one makes a fuss. And this isn't the "just show up and smile" cameo like the kind we saw in the pilot of Grandfathered (2015) wherein Don Rickles, Lil Wayne, and Dion Sanders appeared just to take a picture with John Stamos. (I know what you're thinking: it is animated, so how could a guest star have a non-speaking cameo? Hello, South Park (1997).) But guest spots on these animated shows have ranged from a few scenes to multiple episodes across multiple seasons. What draws A-list names to shows that don't get nearly as much publicity as a run-of-the-mill live action sitcom?
The very first season of Rick and Morty, which airs on Adult Swim (Cartoon Network's late night R-rated cousin) was able to get Dana Carvey, David Cross, Alfred Molina, and John Oliver. They clearly had to raise the bar for Season 2, so they got Christina Hendricks, Patton Oswalt, Stephen Colbert, and Werner Herzog... yes, Werner Herzog! As "Alien in Wheelchair," a famous interplanetary diplomat, he delivers a 30-second monologue about how the human race is obsessed with penises (that's probably something Werner Herzog would do anyway).
FX's Archer (2009) boasts an equally impressive guest list which includes Christian Slater, Fred Armisen, Jeffrey Tambor, John Hamm, Ron Pearlman, Bryan Cranston, Kenny Loggins, and Burt Reynolds. Still, Bob's Burgers could put both of those shows to shame with not only its pedigree of names, but the sheer number of stars it employs on top of its accomplished main cast. To name a few: Sarah Silverman, Kevin Kline, Zach Galifinakis, Aziz Ansari, Key and Peel, Ken Jeong, Nick Offerman, Jim Gaffigan, Bob Odenkirk, Carl Reiner, and Cyndi Lauper have all done one or more episodes on Fox's hit cartoon.
To be fair, going through the guest lists of each show, there are some overlaps, such as David Cross and Matt Walsh, who (perhaps) aren't the busiest people and have a little extra time to do something like this. The same might go for Kenny Loggins, Burt Reynolds, Cyndi Lauper, Kevin Kline, Patton Oswalt, and Werner Herzog (although who ever knows what's going on with Werner Herzog?). But if the argument is that they have the free time, why not do guest spots on live action shows? Most live action shows could pay more. Still, one imagines that for someone like Cyndi Lauper a little extra cast for one guest spot isn't crucial. So is it simply that animation takes less time? Obviously, lending your voice to Family Guy (1999) doesn't take as long as filming a whole episode of 30 Rock. Perhaps the number of people doing guest spots on animated shows reflects the general desire of celebrities to do guest roles, even when they don't have that much time or just don't want to go through the hassle of committing to a whole live action show.
Rick and Morty, despite its relative success, is still somewhat hidden in the ether, so you wonder to what extent the creators reach out to celebrities to be on the show or whether some might not be huge fans who seek out the experience themselves. Whatever way it happens, the prevalance of famous guests on Rick and Morty continues to be striking. None of these guest stars are voice acting veterans (those are usually the stars of the cartoon), and many times viewers can't even tell that the person voicing a guest character is a well-known celebrity that they might love. The mere fact that animation doesn't take up that much of a person's time can't be the only reason why so many actors choose to do this, can it? There must be something inherent to the medium of animation that many of these guest celebrities appreciate working in -- and only in cartoons can someone get to a play an interplanetary diplomat named Shrimply Pibbles. Whatever the actors' reasons, we can only hope that the pedigree of stars these cartoons attract over time will draw more recognition and respect for animated shows from the general public.