The actress, who plays Sansa Stark on the show, disclosed the closing sequences after he “figured it out” as a result of being on set so much with her.
Speaking to Entertainment Tonight she said: “Joe was on set most of the time so he kind of figured it out and he was like, ‘Can you fill me [in]?’
“And, he signed an NDN, NDA and everything so it’s all good.’”
Turner’s comments come days after she admitted to having no problem with her co-star Kit Harington being paid more than her.
She said Harington, who plays Jon Snow, deserved the additional profits as a result of his gruelling late night shoots and overall bigger role on the show.
“Kit got more money than me, but he had a bigger storyline,” she told Harper’s Bazaar. “And for the last series, he had something crazy like 70 night shoots, and I didn’t have that many.
“I was like, ‘You know what…you keep that money.’”
The gripping show, which ends next month after eight seasons, has become known for its secrecy around spoilers.
Maisie Williams appeared to drop a mammoth spoiler about her character Arya Stark dying in episode two, which she later revealed was a savage April Fools’ Day joke.
Williams was visibly close to tears as she discussed the impending finale on The Tonight Show starring Jimmy Fallon.
Appearing to “accidentally” let slip the news, she told Fallon:” During shooting, the final days were really emotional saying goodbye to all the cast. I found out that Arya died in like the second episode..”
A horrified Williams then slammed her hands over her face before running off stage.
Fallon followed her before jumping out at the audience and revealing the prank.
Game Of Thrones will return for a final series on April 15, simulcasting with the US at 2am. Episodes will be repeated at 9pm on Sky Atlantic.