In an interview with Radio Times, Williams was asked if she was trying to secure a bridesmaid position in Turner and the DNCE singer's future wedding.
"Oh, already got it. Yeah, it's very, very exciting. It's kind of bizarre though," she said.
The 20-year-old, who plays Arya Stark, sister to Turner's Sansa Stark, on the hit HBO show, revealed that Turner has not started the planning process for the wedding.
"We're waiting 'til this season's done until we get into it. But I think she's already letting her little heart wander and imagine," Williams said.
Turner and Jonas announced their engagement on Instagram in October 2017.