Tamzin Merchant was originally cast in the role of Daenerys Targaryen on Game of Thrones, a decision announced by HBO on 20 August 2009. She played the role in the pilot episode filmed in late 2009, with her scenes being filmed in Morocco. However, rumors began circulating that Merchant was departing the project after casting sides for the role of Daenerys reappeared on 25 March 2010, after the series was picked up. Her departure was confirmed on 29 March and her replacement, Emilia Clarke, was announced on 21 May. No official reason has been given for her departure, but George R.R. Martin reports that there were no problems with her performance in the role. Merchant went on to appear in the BBC miniseries The Mystery of Edwin Drood.
Ironically, while Merchant played Henry VIII's fifth wife Catherine Howard, in The Tudors, Natalie Dormer, who played his second wife Anne Boleyn, was later cast in Season 2 in the major role of Margaery Tyrell. As is well-known from the rhyme, "divorced, beheaded, died; divorced, beheaded, survived", both Howard and Boleyn were ultimately beheaded.
Martin himself has expressed on his blog his own admiration for Merchant's performance as Katherine Howard.