"I already got it," Williams said of the position. "It's very, very exciting. It's kind of bizarre, though."
"We'll wait [to plan] until the season's done," she added, referencing production on Season 8. "I think she's already letting her little heart wonder and imagine."
Turner and Jonas, an American singer, got engaged in October after nearly a year of dating. Williams was among those to congratulate the couple at the time on social media.
"Congratulations to my beautiful girlfriend on her engagement," she wrote on Instagram. "This is just one of the MANY life milestones we'll share. Love you both."
Williams and Turner play sisters Arya and Sansa Stark on Game of Thrones, which will return for an eighth and final season in 2019. The HBO series co-stars Kit Harington, Emilia Clarke and Lena Headey.