If you look through all the heads when you customize your character at the very beginning of the game, every head has the basic head and one random other one. Except for the star heads. Star head characters (Also known as special characters) are only available through limited distrubution events. The only other way to get one are to play Castle Crashers and/or Alien Hominid HD and get an achievement to get the main character's head to use.
Note - Only the first Star Head character has an actual star shaped head.
Getting Star Heads from Distrubutions
At The Behemoth Development Blog, they recently announce Furbottom Fridays (FF). Every Friday, they upload a new playlist Arena mode, and every other Friday, they upload two new playlists for the Single Player and Co-op modes. Also, if you beat the Single Player feature (exculding encores), you get a head. Same goes for Co-op. You don't get any Special Heads for completing the Arena playlist.
Getting Star Heads from Other Games
If you happen to own any of the other games made by The Behemoth (Alien Hominid HD and Castle Crashers) and have at least one acheivement in any of those games, you get the head of the main character of that game (An Alien Hominid head for Alien Hominid HD, and a Castle Crasher Knight head for Castle Crashers). WARNING: If you trade these heads via Xbox Live, you CANNOT get them back unless you get them from someone else. One way to avoid this is to make a random account and get the special head on that account, in case you accidently trade yours away.
Also, if you get at least one achievement in BattleBlock Theater, you unlock Hatty Hattington, the Gem Sword, and the Golden Whale animal orb in Castle Crashers. (Unconfirmed)
List of Star Heads Distributed
Note- Names in Italics are prisoners obtained by beating Single Player, and names in Bold are prisoners obtained by beating Co-op (Both formatting means you can unlock it by either doing Single Player OR Co-op. You can still play both playlists if you want.) Normal formatting means you just had to open Furbottom's Features to obtain it. Pictures may be added at a future date. A star (*) means that the head is animated.
- April 3rd, 2013 - April 9th, 2013: Behemoth Chicken
- April 10th, 2013 - April 18th, 2013: Purhamm Furbottom
- April 19th, 2013 - May 2rd, 2013: Donuts & Winston
- May 3rd, 2013 - May 16th, 2013: Toast
- May 10th, 2013 - May 16th, 2012: Rose (Special Mother's Day Head)
- May 17th, 2013 - May 30st, 2013: CatControl
- May 27th, 2013 - June 3rd, 2013: Behemoth Chicken (Re-released for The Behemoth's 10th anniversary)
- May 31st, 2013 - June 13th, 2013: LavaLamp*
- June 7th, 2013 - June 13th, 2013: Donuts (Re-released for National Donut Day on June 7th)
- June 14th, 2013 - June 27th, 2013: Tiny Monkey