When Robert De Niro heard his co-star, John Cazale, might be dropped from the production of ‘The Deer Hunter’ because the actor’s terminal bone cancer would make him uninsurable, De Niro covered it himself. Cazale ended up passing away shortly after production.

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I think it is safe to say that John Cazale was one of the greatest character actors in movie history. He appeared in five films between 1973 and 1979, all of them nominated for Best Picture: (The GodfatherThe ConversationThe Godfather Part IIDog Day Afternoon, and The Deer Hunter.)  Cazale was diagnosed with lung cancer in 1977 and died of complications to it in 1978. 
His close friend and collaborator, Israel Horovitz, published a eulogy in the Village Voice on March 27, 1978. It said the following:
John Cazale happens once in a lifetime. He was an invention, a small perfection. It is no wonder his friends feel such anger upon waking from their sleep to discover that Cazale sleeps on with kings and counselors, with Booth and Kean, with Jimmy Dean, with Bernhardt, Guitry, and Duse, with Stanislavsky, with Groucho, Benny, and Allen. He will make fast friends in his new place. He is easy to love.
Much has also been said about the unique bond he had with Meryl Streep, they were also dating, as she stayed with him until his very last breath at the hospital,  Al Pacino has said: "I've hardly ever seen a person so devoted to someone who is falling away like John was. To see her in that act of love for this man was overwhelming."

Cazale worked through "The Deer Hunter" knowing he only had months to live. 

Now comes this nifty little story from Vintage, which claims that Cazale's "Deer Hunter" co-star Robert De Niro threatened to leave production if producers were to let the dying actor go due to insurance policies:

"During the filming, John was growing sicker and sicker, and the producers tried everything they could to nullify his contract and replace him. But Robert De Niro and Meryl threatened to walk away if they did, so in the end, Cimino rearranged his schedule and shot John’s scenes first. To keep up with the growing medical bills, Meryl took up a role for a nine-hour television drama, Holocaust, right after she was finished with her scenes in The Deer Hunter. Unfortunately, the series was mostly shot on real-life locations in a concentration camp in Austria so she had to leave for a while. “I was going crazy, John was sick, and I wanted to be with him,” she said later."
"While she was away for two months, his old friend Al Pacino took him to radiation treatments and De Niro took care of the insurance policy. Just as she got back and things looked as if they were going to be fine, John was once again admitted to the Memorial Sloan Kettering Cancer Center, and on 12th of March, 1978, a doctor woke her up early in the morning with the words, “He’s gone.” Reports say that Streep was so reluctant to let him go, that she threw herself on his chest, begging him to wake up. And as she was sobbing over him, he opened his eyes for a brief second and with the weak voice of a dying man, said: “It’s all right, Meryl, it’s all right,” after which he left this world for good."