Jonathan Pryce is a hugely versatile actor, well-versed in theatre and smaller film and TV roles before achieving his breakthrough as the lead actor in Terry Gilliam's 1985 cult classic Brazil. He played the main villain in the James Bond movie Tomorrow Never Dies as well as playing Juan Peron in Evita. Numerous film and TV roles on both sides of the Atlantic followed, including a recurring role as Governor Swann in the Pirates of the Caribbean movies.
On Game of Thrones he will be playing the 'High Sparrow'. The High Sparrow is a wandering septon (a priest of the Faith of the Seven) who preaches to the smallfolk of the Seven Kingdoms. Sickened by the ravages of the War of the Five Kings, he travels to King's Landing to seek greater help from the High Septon and the Iron Throne in protecting the people. Several players in the capital seek to use his arrival for their own ends.
Alexander Siddig is best-known for his seven-year stint as Dr. Julian Bashir on Star Trek: Deep Space Nine. He has appeared in films such as Syriana, Kingdom of Heaven and Clash of the Titans, and TV shows including 24 and Atlantis (having multi-episode arcs on both).
On Game of Thrones Siddig will be playing Prince Doran Nymeros Martell, the ruling Prince of Dorne. One of the most powerful men in the Seven Kingdoms, Doran is the older brother of Prince Oberyn, the Red Viper, but ill health has kept Doran at home in his castle of Sunspear overlooking the Narrow Sea for several years. Unlike his younger, headstrong and more fiery brother, Doran is noted for being thoughtful, slow to anger and keen on keeping Dorne out of war. The other rulers of the Seven Kingdoms do not consider him a major threat, believing his is afraid to take bold action. However, his peaceful policies also have their opponents within Dorne.
The other actors announced at Comic-Con (and rounded up at Winter is Coming) are:
Toby Sebastian as Prince Trystane Martell, Prince Doran's son.
Nell Tiger Free as Princess Myrcella Baratheon (replacing Aimee Richardson).
DeObia Oparei as Areo Hotah, Prince Doran's bodyguard and captain of guards.
Enzo Cilenti as Yezzan zo Qagazz, a slave master of Yunkai.
Jessica Henwick as Nymeria 'Nym' Sand, one of Prince Oberyn's daughters, the Sand Snakes.
Rosabell Laurenti Sellers as Tyene Sand, another Sand Snake.
Keisha Castle-Hughes as Obara Sand, another Sand Snake.
There is looming controversy in the decision to replace Aimee Richardson, who was popular amongst fans for her amusing Tweets and engagement with them at conventions, and most notably the official HBO description of Prince Trystane as Doran's heir. This heavily suggests that both Princess Arianne Martell, the heir to Dorne in the novels, and Prince Quentyn (Trystan's older brother) have either been removed from the TV series or somehow delayed until later seasons.
There is also some surprise at a lack of any indications that characters such as Euron and Victarion Greyjoy, Young Griff or Jon Connington will appear this season, although it is possible that further announcements will be made down the line. It is also possible that Season 5 will see the most drastic changes yet to GoT's story, as entire plots and subplots from the novels are discarded.
Season 5 of Game of Thrones is now shooting in Northern Ireland, Croatia and Spain (no Iceland this year, apart from establishing shots) and will air in the USA and UK in March or April of 2015.