Marilyn Monroe died during the filming of “Something’s Gotta Give”.
(I looked this up on the IMDB to make sure. She was fired from the film but then rehired.)
Not exactly an actor, but there’s also Christine Chubbuck, a news reporter who committed suicide on air during a live news broadcast.
Her last words were:
Zev Steinhardt
Three extras were killed during the filming of the flood scene in Noah’s Ark. The footage was used in the finished film, but it was a crowd scene and you probably couldn’t see any of them.
Fearless, yes. But it appeared to kill her career. When was the last time Marsha appeared in anything?
And was replaced by Keanu Reeves?
Anita Mui died right before filming was to start on House of Flying Daggers. Out of respect for Mui, Yimou Zhang chose not to recast her part, and shuffled the plot around, leaving a big hole in the story where her character was to have been. More info here.
LOL Otto
I guess I should learn to be more precise in my postings OR include a link to the actual movie:
And I think we’ll agree that Keanu Reeves is NO Marilyn Monroe. 
I’ve also heard accounts that the reason the helicopter flew too low and crashed was that the stunt pilot was told to fly down a narrow canyon, but the director conveniently forgot to tell the stunt pilot that he would be setting off explosive pyrotechnics along the walls of said canyon as the helicopter passed by. (Movie directors are not known for being the sharpest brick in the chandelier when it came to aviation safety.)
Marty Feldman died in the middle of Yellowbeard, and so they rewrote the film so that his character fell into a vat of acid. It made a bad movie just that much worse.
Didn’t she retire due to ill health?
Alice Pearce (the first Mrs. Kravitz) died during the run of Bewitched and was replaced by Sandra Gould. Marion Lorne (Aunt Clara) also died during the run of the show, though being a recurring character (27 episodes to be precise) she wasn’t as hard to write out; they just never referenced her again and brought in another bumbling witch (Alice Ghostley) and at least one of the exact same scene for scene plotlines (the babysitting for the client’s kids episode).
Bea Benadarat died after a long battle with cancer during the run of Petticoat Junction (on which she was a regular) and Beverly Hillbillies (on which she had a recurring part). On Petticoat her character would sometimes make telephone calls or be seen from the back entering- Benadarat would record the voice for those performances from her sickbed.
Zara Cully (“Mother Jefferson”) died during the run of The Jeffersons. Her death was referenced in the next season but no episodes were devoted to it.
Thanks for jogging my '60s sitcom memory.
Barbara Pepper died in 1969 and was replaced by Fran Ryan as Doris Ziffel (Arnold’s Mom) on Green Acres.
Ann Sheridan died in 1967 while filming Pistols ‘n’ Petticoats and the show was cancelled after that season.
Bewitched had such a turnover in cast (two Darrens, two Mrs. Kravitses, two Louise Tates) that they actually recycled several scripts with the new cast members.
. . . Not to mention that both Paul Lynde and Maurice Evans used to steal Agnes Moorehead’s wig and occasionally play Endora . . .
I want to say Pearl White of the Peril’s of Pauline (silent versions) but when Eve didn’t mention it, I question if I remember it correctly.
I also seem to remember that an actress doing the same film in the 1970s was killed doing her version also.
I do know that Betty Hutton doing the role in the 1940s was not killed.
Nope, Pearl White did suffer a back injury while filming Pauline, and that led to her eventual reliance on painkillers and her death in 1938. But she went on to make some 40 films after Pauline, retiring in 1924.
A low-budget Perils of Pauline starred Evalyn Knapp in 1934; Miss Knapp died in 1981 at the age of 73.
Betty Hutton, who starred in the Pearl White bio-pic in 1947, is still alive and well at 84.
Kooky '60s starlet Pamela Austen (The "Dodge Rebellion! Girl) starred in a high-camp aren’t we cute? version in 1967. The 64-year-old starlet does not seem to have worked since a walkon in a 1988 low-budget film called The Dressmaker.
Isn’t she the one who wound up living in a indigent and living in a homeless shelter some years ago? If so, I hope she’s recovered since then. (Of course Patricia Neal isn’t indigent but lived under a vow of poverty in a convent for some while in recent years, in part to sort out her life after a divorce and to atone for an abortion she had early in her career [she’s majorly anti abortion now].)
I think I remember two episodes in which Endora splits Durwood (one with York and one with Sargent) into two men, one of them a workaholic and the other a doting husband. As memory serves, Samantha was even pregnant in both episodes, but by different Dicks.
Puh-lease!