Thora Birch has joined "The Walking Dead" for Season 10. "Thrilled to be along for the wild ride!" the "American Beauty" and "Clear and Present Danger" alum tweeted. The actress will play Gamma, "a sentry fiercely protective of Alpha," AMC said in a press release. Samantha Morton plays Alpha, the leader of Whisperers -- a group of zombie-apocalypse survivors who camouflage themselves with the decomposing flesh of the dead. Starring Norman Reedus, Danai Gurira, Melissa McBride, Kary Payton, Seth Gilliam, Ross Marquand, Josh McDermitt, Dan Fogler and Jeffrey Dean Morgan, "The Walking Dead" will return for its 10th season on Oct. 6. Gurira, who plays Michonne, announced at San Diego Comic Con on Friday that she is leaving the series this season.