"I had such great hope for him," Gibson, who co-starred with Ledger in the 2000 film "The Patriot," told People.com. "He was just taking off and to lose his life at such a young age is a tragic loss. My thoughts and prayers are with him and his family."
Kidman, whose friend Naomi Watts dated Ledger for years, echoed Gibson's sentiment.
"What a tragedy. My heart goes out to his family," she said.
"Heath Ledger was a courageous actor, and a great soul. He gave us the gift of sharing his fearless and beautiful love -- of his craft, and of all who worked with him -- for which all of us will be eternally grateful," said James Schamus, CEO of Focus Features and producer of "Brokeback Mountain," the 2005 film that earned Ledger an Oscar nomination.
Ledger was pronounced dead at 3:45 p.m. EST in a New York apartment.
Police said they didn't suspect foul play.
"There were some drugs found around the body, prescription-type drugs," a police spokesman told People.com.