When NBC's breakout hit This Is Us returns for its second season this fall, it will include a very familiar face. But a very unexpected face!

Former action movie hero Sylvester Stallone will guest star in an episode, This Is Us creator Dan Fogelman said at the Television Critics Association summer press tour on Thursday.

Stallone will appear as a costar in a period war film that Justin Hartley's character Kevin Pearson is in, and he will be like a "father figure" to him, according to Fogelman. The film itself isn't titled yet — he said they're still writing the episodes — but it's a "really beautiful part and monologue for him about life."

Fogelman grinned ear to ear as he talked briefly about the episode, saying he was very excited for Stallone's appearance.

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"We needed a big star for this movie," Fogelman said. "And we're really excited for the script."

Fogelman told TV Guide that Kevin will play a young soldier in a war film, and Stallone will play a higher-ranking official who works with him. Given Kevin's past insecurities about being a leading man, Stallone will help Kevin gain the mojo he needs to grow as an actor and, because this is This Is Us, a human being. "He learned about what happened to Kevin in his life with his own father, and [Sly] is talking about his life, experience and loss and aging and the business. It helps in a few ways. And it sends us down a few rabbit holes too."

As for Hartley, he's still pinching himself that he gets to act alongside an icon.

"He's very gracious. He's awesome. He's on board and ready to go. I can't wait. I can't believe I get to work with Sylvester Stallone," he exclusively told TV Guide after the This Is Us panel. "It's still kind of ridiculous for me to think about it...I'm pinching myself because it doesn't get any bigger than this."

Stallone's appearance was in part facilitated by This Is Us star Milo Ventimiglia, who costarred with Stallone in the 2006 film Rocky Balboa.

This Is Us returns Tuesday, Sept. 26 at 9/8c.