The Scottish actor was spotted on set, prompting speculation he was doing more than paying a friend a visit, and now he has come clean.
"I'm enjoying that, yes," Graham confirmed when asked about the HBO show. "I'm really having a lot of fun. We've just started. It's a very big project, so we're getting to know each other."
"They're lovely people. It's great, but I can't tell you too much. I can't tell you really anything about the story! But it involves a lot of dragons."
Production on "House of the Dragon" got underway last month (April 2021), and George R.R. Martin, Ryan Condal and Miguel Sapochnik's epic will debut some time next year (2022), starring Paddy Considine, Olivia Cooke, Matt Smith, Emma D'Arcy, Fabien Frankel, Rhys Ifans, Steve Toussaint, Eve Best and Sonoya Mizuno.
The drama takes place 300 years before the events of "Game of Thrones". Considine leads the cast as King Viserys Targaryen, supported by Cooke as Alicent Hightower, and D'Arcy as Princess Rhaenyra Targaryen. It is not clear which character McTavish will play.
While it is still unclear what character McTavish will fill in, fans speculated that he would take on the role of Harrold Westerling after he was spotted on the Cornwall set of the prequel series. About the character itself, a report from the Radio Times described him as "the commander and longest-serving member of the kingsguard who still remembers the lustre of the storied Seven Knights."
"A once powerful knight and a successful military background, so a man with presence and status. Given the responsibility to the king to watch over and guard the princess," the description continued. "In the current regime, where widespread peace has led to complacency and indulgence, he feels something of a loss of purpose, but nevertheless serves the king with a fierce, if quiet, loyalty."