who are the main characters?
Rick Grimes, Dale Horvath, Carl Grimes, and a few others.
Judge, Jury, Executioner
 In this episode, Rick Grimes (Andrew Lincoln) and his group opt to execute Randall (Michael Zegen), much to the frustration of Dale Horvath (Jeffrey DeMunn). 
what is judge, jury, executioner? An episode of the series "The Walking Dead". when was it first aired? March 4, 2012
wiki:19725872
Judge, Jury, Executioner [SEP] Introduction
"Judge, Jury, Executioner" is the eleventh episode of the second season of the post-apocalyptic horror television series "The Walking Dead". It originally aired on AMC in the United States on March 4, 2012. In this episode, Rick Grimes (Andrew Lincoln) and his group opt to execute Randall (Michael Zegen), much to the frustration of Dale Horvath (Jeffrey DeMunn). Dale fears that the group is losing their humanity, which prompts him to persuade some of the group members to protest against the consensus. Meanwhile, Carl Grimes (Chandler Riggs) behaves recklessly and carelessly, going as far as to steal Daryl Dixon (Norman Reedus)'s gun and harass a walker, which will ultimately initiate grave consequences for the group.