The British actor is best known as the no-nonsense cop on The Wire and was first choice for the role of Mance Rayder, leader of the Wildling forces and self-proclaimed King Beyond the Wall. West turned the role down due to family commitments, which is fair enough, but also something he must look back on with a tinge of regret now. Ciaran Hinds was able to step in, his wise head on a battle weary body is the perfect foil for the character of Rayder. It's hard to imagine how the character would have come across if West had remained in the role being much younger and slightly more civilised in appearance.
Derek Halligan as Alliser Thorne
Alliser Thorne, who is a major player in the Night's Watch family has been brilliantly portrayed by Owen Teale. We know him best for his bravery when fighting to protect Castle Black, despite being the nasty piece of work that gave Samwell Tarly the nickname “Lord Piggy” and is always pretty awful to all the new recruits who are unfortunate enough to end up serving at The Wall. His relationship with Jon Snow is a standout storyline throughout each of the series to date and Teale plays the part so well, he's really made viewers hate the character
However, it was Derek Halligan who was originally set for the role. Halligan is so different in appearance to the scrawnier, more vicious looking Teale and the role surely would have played out very differently.