"Misty was a beautiful soul and a phenomenal actress who had seemingly never-ending reserves of talent," Daniel Barnz, her director in the upcoming comedy-drama Cake, tells PEOPLE. "She was an open and giving person. I'm heartbroken over her death."
Meryl Streep, who costarred with Upham in the 2013 drama August: Osage County, also expressed her sadness. "So so sad to hear this," the Oscar winner said in a statement. "All our thoughts are with her family and with her beautiful spirit."
Fellow August costar Juliette Lewis tweeted her grief, saying, "I can't sleep. Too much darkness. I feel so sad about Misty. I know she has more to say. And about how she died. #RIPMistyUpham."
Upham, who also appeared in 2012's Django Unchained, had been missing since Oct. 6. After police in Auburn, Washington, found her body at the bottom of a 150-ft. embankment on Thursday, her father, Charles Upham, confirmed her identity to authorities. The cause of death has yet to be determined.
The acclaimed Native American actress, who left Montana's Blackfeet reservation as a child and grew up in Seattle, was "an amazing person, a wonderful friend," her friend, Craig Jordan, told PEOPLE, "and a tireless activist and an incredibly gifted actress."
Barnz recalls Upham's commitment and work ethic on the set of Cake, which costars Jennifer Aniston and Felicity Huffman. "When we were making the movie, she had a last-minute emergency the day before we were to shoot, and she drove through the night to make sure she arrived on time," he says. "When she did, having not slept in 24 hours, she still brought her all. It was an emotional scene, and take after take she delivered.
"Everyone in the cast – Jen, Felicity – was blown away by her."
Reporting by ELIZABETH LEONARD and MELODY CHIU