The "author" and Grunkle Stan are seen in the "A Tale of Two Stans" episode of Gravity Falls
Disney XD
June 12, 2015 12:00 PM EDT
J.K. Simmons during a recording session for Gravity Falls.
Guy D'Alema—Disney Channel

Disney XD’s Emmy-award winning Gravity Falls left viewers with a major cliffhanger in its mid-season finale when it was revealed that Grunkle Stan (voiced by series creator Alex Hirsch) has a twin brother—who is the mysterious six-fingered author of the journals that have led twins Dipper (Jason Ritter) and Mabel Pines (Kristen Schaal) on supernatural misadventures in the fictional town of Gravity Falls, Oregon.

The previously unseen brother will be voiced by Academy Award winner J.K. Simmons, in a recurring role on the series. Simmons isn’t a newcomer to voice work, having previously voiced characters on shows like Kim Possible; more recently, he served as the voice of Tenzin in Legend of Korra.

His debut episode will be “A Tale of Two Stans,” premiering on July 13 on Disney XD. The episode will pick up where the mid-season finale ended and will also reveal the name of the Grunkle’s brother.

Previous guest stars on Gravity Falls have included Nick Offerman as the voice of a secret agent and Neil deGrasse Tyson as the voice of a super-intelligent pet pig.

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