Most of their music videos also include a host of friends from bands on Pete's record label Decaydance.
Seth Green and Michelle Trachtenberg (both from Buffy the Vampire Slayer) show up briefly at the funeral scene in "Arm's Race", and the chick who plays Cerie on 30 Rock is the mean girl in "Dance, Dance"
Brendon Urie and Spencer from Panic! at the Disco make an appearance in the video for "What a Catch, Donnie".
The "A Little Less Sixteen Candles" video has William Beckett and Mike Carden from The Academy Is..., Travie McCoy from Gym Class Heroes, and Brendon Urie and Spencer Smith from Panic! at the Disco all as various vampires from different clans.
Cassadee Pope, then of Hey Monday and later on of The Voice, cameos in the "America's Suitehearts" video.
The music video for "My Songs Know What You Did In The Dark (Light Em Up) consists entirely of 2Chainz and two other women setting fire to Fall Out Boy's discography and memorabilia.
Big Sean shows up in the video for "Alone Together", as set up for his appearance in The Mighty Fall.
Tommy Lee is the Devil in the video for "Death Valley", along with SOFI (aka SomeOtherFemaleInterest, known for her collaborations with deadmau5 and needing a ladder), as a turncoat member of the organization hunting down the boys that's willing to help them out. While this is Tommy Lee's first time in a Fall Out Boy video, this isn't SOFI's. She appears alongside Butch Walker briefly in the video for "This Ain't A Scene, It's An Arms Race" And last but not least, Elton John AS God himself!
William Beckett sings in "Sophomore Slump or Comeback of the Year".
The other screaming voice on "I Slept With Someone in Fall Out Boy and All I Got Was This Stupid Song Written About Me" is Chad Gilbert of New Found Glory.
In "7 Minutes in Heaven (Atavan Halen)" and "20 Dollar Nosebleed", Brendon Urie lends his vocals. However, it may be hard to pinpoint his parts as he and Patrick have similar voices and singing styles.
In "What a Catch, Donnie", a few guest stars lend their voices to sing a few of their greatest hits.
You know the second singer in "Save Rock and Roll"'s title track? Its only Elton Fucking John!
Courtney Love is featured on "Rat a Tat", Big Sean on "The Mighty Fall", and Foxes on "Just One Yesterday".
A non-musical example: Tara Strong narrates the opening of the "Miss Missing You" video for The Youngblood Chronicles. Her opening spiel is actually reversed text on the back of the Save Rock and Roll album.
Magnum Opus Dissonance: Patrick had a lot of high hopes for Infinity on High, his role in the album was mainly a response to thinking that From Under The Cork Tree' wasn't entirely deserving of the Grammy nomination it got. He explains more about it in the liner notes for the Believers Never Die greatest hits compilation.
Old Shame: Evening Out With Your Girlfriend, mainly due to the recording circumstances and the intent for it not to be released. Whenever the band discusses their history, Evening Out almost never comes up. "Growing Up" was acknowledged on "What a Catch, Donnie" and it made an appearance on the greatest hits compilation released before their hiatus, mainly due to it being the band's first song.
One of Us: One of their first tours in support of From Under The Cork Tree was the Nintendo Fusion tour, they did two alternate reality games for Folie a Deux and their Greatest Hits album, did an entire photoshoot with lightsabers, and on one of their blink-182 reunion support dates, they did a set dedicated to LOST.
In terms of One Of Usness...Andy, Pete, Patrick, Joe. Arguably.
Andy is a big comic book fan, Patrick has a preference for oldschool video games (when he gets a chance to play), and Joe used to go into long, elaborate blog posts about Star Wars back in the Take This To Your Grave days.