The Twin Peaks actor was set to star alongside Josh Whitehouse in a project from showrunner Jane Goldman (Rebecca), and had finished shooting the pilot episode last year in Northern Ireland.
George RR Martin, the author of the A Song of Ice and Fire book series, on which Game of Thrones is based, was working with Goldman as the prequel’s co-creator.
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In October, it was revealed that HBO had cancelled the show.
“I’m sorry,” Watts said of the news, addressing fans for news.com.au. “I feel your pain. I equally got into it. I wasn’t a huge fan and hadn’t seen the shows until I was hired and then completely binged everything within the space of a couple of months and it’s just wonderful.”
She added: “It’s a deep shame, it would have been great fun. But I am not allowed to give anything away I’m afraid.”
The prequel was meant to be set thousands of years prior to the events depicted in Game of Thrones.
Another Game Of Thrones prequel is currently in the works, titled House Of Dragon. Paddy Considine has been cast as King Viserys Targaryen in the series, which will be set 300 years before the events of the original show.