Actress Zosia Mamet stunned her Girls castmates during a recent TV interview by revealing her grandfather wrote The Sound Of Music.

The star and her castmates were appearing on Inside the Actors Studio, which aired in America on Thursday night (09Feb17), when she started talking about her family.

And, after chatting about her dad, Pulitzer Prize-winning writer David, and her actress mom Lindsay Crouse, she opened up about her maternal grandfather, Russell Crouse, revealing he "wrote The Sound of Music".

Her castmates freaked out, with Lena Dunham and Jemima Kirke stating, "I did not know that", while musical theater obsessive Allison Williams said, "What? Zosia Mamet... That is... This is weird."

The actress then joked, "I saved that for right now."

Crouse adapted Maria Von Trapp's book for the stage with Howard Lindsay.

During the TV chat with Inside the Actors Studio host James Lipton, Mamet also revealed she and her mother are back on speaking terms after she kicked the troubled teen out of her house at 17.

"We actually, in the not so recent past, have started to form a really wonderful relationship again," Zosia said. "She's very thrilled (about my success)."