Canadian director Jean-Marc Vallée died suddenly on Christmas day at the age of 58. Hollywood stars, in particular those who worked with him, are now expressing their grief on social media.

Deadline reports that Vallée passed away in his cabin outside Quebec City, leaving his family and close advisers in shock. Sources are suggesting that a heart attack was the cause of Vallée’s sudden death, though this information has yet to be officially released. Vallée was a fitness fanatic who did not drink alcohol. Moreover, he was a practitioner of the  Wim Hof Fitness Method, which focuses on breathing exercises, meditation, and exposure to extremely cold temperatures.

Several people have expressed grief over Vallee’s death, including Canadian Prime Minister Justin Trudeau. Actors who worked with Vallée are also mourning his loss. Matthew McConaughey and Jared Leto both won Oscars for Vallée’s film Dallas Buyers Club, and both stars have shared their sadness on Instagram. McConaughey remembered Vallée as one who “didn’t romanticize life so much as he saw life romantic.” Leto remembered Vallée as “A filmmaking force and a true artist who changed my life with a beautiful movie called Dallas Buyers Club.”

Actress Reese Witherspoon, who worked with Vallee on Big Little Lies, also expressed heartbreak on Twitter regarding the director’s death. Vallée served as both director and executive producer for that series.

Earlier this year, Vallée had signed a deal with HBO Max. Representatives were excited to tell more stories with the director. However, we may never see these storied due to the director’s death.