The Danza: Cecil, voiced by Cecil Baldwin, and Kevin, voiced by Kevin Free. As noted below, both have ties to the Neo-Futurists improv group which uses cast members' real names in sketches, likely the inspiration for the podcast.
"It was very strange for me, I think it was in the fourth or fifth episode, when I read the script and found out the character’s name is Cecil—which, of course, is my name," Baldwin said. "And I wrote Joseph an email and I was like, ‘So am I playing myself or is this supposed to be autobiographical or am I playing a character who just happens to be called Cecil or is it just a coincidence or is it kind of all three together?’ And Joseph was like, ‘Yeah, it’s really everything.’" —from an interview
If you're a Something Awful Goon, yep, this is the same Joseph Fink that used to write front-page articles.
Jeffrey Cranor (Co-writer), Dylan Marron (Voice of Carlos), Cecil Baldwin (Voice of Cecil) and Kevin Free (Voice of Kevin) are fixtures in the New York performance art scene, with ties to the Neo-Futurists.
In episode 30, Dana is voiced by Jasika Nicole - better known as Astrid Farnsworth in Fringe.
In episode 32, Hiram McDaniels (who is literally a five-headed dragon) is voiced by Jackson Publick of The Venture Bros. fame.
One of his heads even sounds suspiciously like The Monarch.
In the L.A. Podcast Festival episodes, James Urbaniak, also of The Venture Bros. fame, fills in with redone ads for older episodes, voicing them as intern Leland and his identical relative who end up getting the Redshirt treatment.
Jossed: One popular fanon for Cecil was having his last name be Baldwin, just like his voice actor. In "Cassette" however, it was revealed that his full name is Cecil Gershwin Palmer.
One of Us: Cecil Baldwin linked to the TV Tropes entry for the show on his Facebook, calling attention to the "Adorkable" description in particular. He often posts fanart of his character by various artists on Tumblr and seems both knowledgeable and delighted by the fan-culture surrounding the show, describing himself as a "Night Vale Geek" in an interview.
According to the aforementioned Facebook, he's also apparently a Whovian!
The Other Darrin: Carlos was voiced by series co-creator Jeffrey Cranor for his appearances in "The Phone Call" and the first performance of "Condos". Commonplace Books announced in December 2013 that Carlos' subsequent appearances would be voiced by Dylan Marron, as a straight white actor playing a queer Latino character had some Unfortunate Implications, especially given the Internet Backdraft described on the YMMV page. Marron debuted in the recorded performance of "Condos".
The Wiki Rule: There is a wiki! It's currently quite small, but listen: no wiki is perfect. It becomes perfect when you learn to accept it for what it is.