HOLLYWOOD — The actor had set his coffee cup down with his left hand in the wide shot and reached for it with his right in the close-up, and Adele Marchetti, watching from her folding chair beside the camera, was the only person on the set who noticed before it cost anyone a day.

Mrs. Marchetti, a script supervisor whose job was to hold a film’s continuities in her head so the finished picture would not fall apart, died September 24 at Providence Saint Joseph Medical Center in Burbank. She was 90. Her niece confirmed the death.

The script supervisor sits where few in the audience ever imagine anyone sitting: at the elbow of the camera, script open, stopwatch running, tracking which hand held the cup, how far the cigarette had burned, whether the door stood open or shut when the scene last cut away. Films are shot out of order and in fragments, and someone has to remember, across days or weeks, exactly how a moment looked the first time so it can be matched the fiftieth. On soundstages and locations across Los Angeles, for sixty years, that someone was often Marchetti.

Adele Fiore was born in Boston in 1934 and came to California in 1957 to visit a cousin. She took a typing job in a production office, learned the work by watching, and by the early 1960s was running continuity on features. Directors requested her by name and kept requesting her for decades, because a good script supervisor saved them from themselves.

“Adele never made you feel stupid for the mistake she’d just saved you from, and there were a lot of them,” said Marguerite Colson, an assistant director who worked with her often. “She’d lean over between takes and murmur, ‘His tie was looser,’ and that was it. No lecture. You fixed it and thanked God she was there.”

She kept her notes in a shorthand of her own devising, columns of times and symbols no one else could fully read, and she was gently territorial about them. She distrusted the video playback that eventually did part of her job, not because it was wrong but because it made people lazy, willing to let a machine watch instead of learning to watch themselves.

Marchetti worked into her early eighties, slowed by the industry’s habits more than her own. She had seen slumps before, she told a younger colleague during the recent downturn, and had learned not to mistake a quiet year for the end of anything.

She is survived by a sister, Philippa Fiore, of Boston; several nieces and nephews; and a great-niece she taught to play cribbage. Her husband, the film editor Sam Marchetti, died in 2009. The couple had no children.

Her scripts, annotated in that private shorthand, went to a film archive at her request. An archivist there admits she may never decode all of it — which would have suited Marchetti, who spent a working life making sure the seams never showed, and saw no reason to start explaining them now.