The original Gregor Clegane had the right sort of imposing look (even if his armour made him look a little like the Tin Man), thanks to veteran stuntman Conan Stevens taking on the role. Sadly, he left the show to work on The Hobbit (a time commitment that stopped any of the cast working on multiple projects at once), before The Mountain really got a great deal to do.
He was replaced by Ian Whyte, but in truth he was a poor stand-in, lacking the hulking physical presence. That clearly wasn't a problem for the final evolution of Clegane - played by Icelandic strongman Hafþór Júlíus "Thor" Björnsson, who is as close to a real giant as you're likely to find.
17. Sarah McKeever (Selyse Baratheon)
The Replacement: Tara Fitzgerald
Back in the opening episode of season 2, Stannis Baratheon's sacrilegious debut saw him align himself with Melisandre and burn the statues of the Seven Gods as a rejection of their ways and a tribute to the Lord Of Light. He was flanked during that ceremony by his wife, played by Sarah McKeever.
She was mostly just a stand-in for that first non-speaking appearance and was replaced by Tara Fitzgerald for the expanded role when Stannis' role became larger. Her reaction to Shireen's death is the only evidence you need that it was the right decision.
16. Callum Wharry (Tommen Baratheon)
The Replacement: Dean-Charles Chapman
You really have to feel for the Baratheon/Lannister kids. Not only were they all killed off in wonderful ways, but two of them were replaced with pretenders just as their roles were getting good.
The first to be shunted out was Callum Wharry, who played Tommen Baratheon when he was little more than background furniture. When it came to him sitting on the throne in Joffrey's wake, it was clearly decided the youngster wasn't up to it and he was replaced with Dean-Charles Chapman, who masterfully captured Tommen's inherent annoyance as well as being a slightly less creepy marriage option for Margaery Tyrell.
15. Roy Dotrice (Grand Maester Pycelle)
The Replacement: Julian Glover
Julian Glover may have been the only face you've ever seen playing lascivious, duplicitous wretch Grand Maester Pycelle in Game Of Thrones, but he wasn't even supposed to play the character.
Inititally, Roy Dotrice was cast - as confirmed by George RR Martin - but his ill-health unfortunately meant he was forced to back out before production had kicked off. His misfortune was to Glover's benefit (and ours, given how well he sold Pycelle's awfulness).
Dotrice would eventually appear in the second season though, playing pyromancer Hallyne for a couple of episodes.
14. Freddie Stroma (Dickon Tarly)
The Replacement: Tom Hopper
Harry Potter fans would have been delighted ahead of season 6 when Cormac McLaggen actor Freddie Stroma was cast to play Sam Tarly's younger, preferred brother Dickon. He became the latest in a long-line of Potter alumni to join the show (a line since joined also by Jim Broadbent), and his previous work as McLaggen definitely served him well to be an unlikeable jock.
Sadly for him, he won't be back for season 7 as Dickon returns thanks to a scheduling conflict. He was cast to play HG Wells in ABC show Time After Time - a role he'd have been mad to pass up - and had to bow out. He will be replaced for Dickon's return by Tom Hopper who previously appeared in Black Sails.