"Do you know what leadership means, Lord Snow? It means that the person in charge gets second guessed by every clever little twat with a mouth. But if he starts second guessing himself, that’s the end. For him, for the clever little twats, for everyone. This is not the end. Not for us. Not if you lot do your duty for however long it takes to beat them back. And then you get to go on hating me, and I get to go on wishing your wildling whore had finished the job."
Alliser Thorne to Jon Snow during the Battle of Castle Black.

Acting Lord Commander Alliser Thorne is a knight and the head of House Thorne, a ranger in the Night's Watch and the master-at-arms at Castle Black, and (in the show), briefly served twice as Acting Lord Commander, and once as First Ranger. Fighting for House Targaryen during Robert's Rebellion, Ser Alliser was present at the Sack of King's Landing, where he yielded and later took the black. He became the master-at-arms at Castle Black, where he trained new recruits: he briefly travelled to King's Landing to show the royal court proof of the Others, and therefore wasn't present for the Great Ranging. In the books, he returned in time for the Battle of Castle Black, and later became a follower of Janos Slynt, right up until he was executed. He is then ordered by Jon Snow to venture beyond the Wall with Dywen and another seasoned ranger, and has not yet returned. In the show, he returned in time for the Battle of Castle Black, where he assumed the title of Acting Lord Commander, until Jon Snow is given the position officially, and he is made First Ranger. He then leads the mutineers in killing Jon Snow, where he temporarily becomes Acting Lord Commander once again until Jon Snow is resurrected, and he is executed, along with the other mutineers.