San Diego Comic-Con News:  (Spoilers Below) During the AMC Panel for The Walking Dead, executive producer David Alpert revealed the role actor Seth Gilliam will play on Season 5--Father Gabriel.
According to Blastr, Alpert confirmed that The Wire star Seth Gilliam will play Father Gabriel, a character that originated in the comics.  The actor’s involvement in Season 5 as a regular was announced in May, but his role was not revealed until now.  
In the comics, Father Gabriel appears in Issue #61 and joins Rick’s group.  The preacher eventually confesses that he initially survived the outbreak by  locking himself in his church and cowardly refusing to let members of his congregation or other survivors inside, letting them be killed by roaming zombies outside his sanctuary’s walls.