There were over 200 guest appearences throughout the entire show.
Here are a few important ones:
Miranda Cosgrove, Snoop Dog & Fabio (Season 2 episode 8 & 9, Big Time Christmas )
Cymphonique Miller as Kat
Cristal Guel as Jay Jay
Danielle Bessler as Shanya
(Season 2 episode 16, Big Time Girl Group)
Elizabeth Gillies as Heather Fox ( Season 2 episode 27 Big Time Secret)
Jordin Sparks (Season 1 episode 16, Big Time Sparks)
Rachel Crow (Season 3 episode 3, Bel Air Rush)
Victoria Justice (Season 4 episode 6, Big Time Tourbus)
Russel Brand & Tom Kenny (Season 2 episode 13 & 14 , Big Time Beach Party)
Jojo Wright (Season 3 episode 2, Big Time Returns)
There were also a few characters that appeared more than once
Malese Jow as Lucy Stone (throughout season 2,3 & 4)
Matt Riedy as Griffin (throughout seaaon 1,2,3 &4)
Obdul Ried as Obdul (throughout season 1,2,3 &4)
Denyse Tontz, Kelli Goss, and Savannah Jayde(throughout season 1,2,3 &4)
Barnett O'Hara as Guitar Dude
(throughout season 1,2,3 &4)
Daran Norris as Buddha Bob (throughout season 1,2,3 &4)
David Cade as Jett Stetson (Season 3 episode 4, Big Time Double Date)
