The Mirror Crack'd

Set in the fictional English village of St. Mary Mead, home of Miss Jane Marple (Angela Lansbury), in 1953, a big Hollywood production company arrives to film a costume film about Mary, Queen of Scots and Elizabeth I with two famous movie stars, Marina Rudd and Lola Brewster (Elizabeth Taylor and Kim Novak, respectively). The two actresses are old rivals who despise each other. Marina is making a much heralded comeback after a prolonged "illness" and retirement (due to what was really a nervous breakdown when her son was born with severe brain damage). She and her husband, Jason Rudd (Rock Hudson), who is directing the movie they are making, arrive with their entourage. When she learns that Lola will be in the film as well, she becomes enraged and vents her anger. Lola and her husband, Marty Fenn (Tony Curtis), who is producing the film, arrive. Excitement runs high in the village as the locals have been invited to a reception held by the film company in a manor house, Gossington Hall, to meet the celebrities. Lola and Marina come face to face at the reception and exchange some potent and comical insults, nasty one-liners, as they smile and pose for the cameras. The two square off in a series of clever verbal cat-fights throughout the movie.
After her initial exchange with Lola at the reception, Marina is cornered by a gushing, devoted fan, Heather Babcock (Maureen Bennett), who bores her with a long and detailed story about having actually met Marina in person during the Second World War. After recounting the meeting they had all those years ago, when she arose from her sickbed to go and meet the glamorous star, Heather drinks a cocktail that was made for Marina and quickly dies from poisoning. Everyone is certain Marina was the intended murder victim. Not only has Marina been receiving anonymous death threats made up of newspaper clippings, once shooting begins on the film she discovers that her cup of coffee on the set has also been spiked with poison, sending her into fits of terror. The police detective from Scotland Yard investigating the case, Inspector Dermot Craddock (Edward Fox), is baffled as he tries to uncover who is behind the attempt on the life of the actress and consequent murder of an innocent woman. Inspector Craddock asks his aunt, who happens to be Jane Marple, who recently injured her foot at the reception and is therefore confined to her home, for assistance. The suspects are Ella Zielinsky (Geraldine Chaplin), Jason's production assistant who is secretly having an affair with him and would like Marina out of the way, and the hotheaded actress Lola Brewster.
The main suspect, Ella Zielinsky, after going to a pay phone in the village where she telephoned and threatened to expose the murderer, is killed by a lethal nasal spray substituted for her hay-fever medication.
Miss Marple, now back on her feet, visits Gossington Hall, where Marina and Jason are staying, and views where Heather's death occurred. Working from information received from her cleaning woman, Cherry Baker (Wendy Morgan), who was working as a waitress the day of the murder, the determined elderly sleuth begins to piece together the events of the fatal reception and solves the mystery. By the time she has collected all the evidence to indicate who committed the crime, however, another death occurs at Gossington Hall, which explains who was the killer: Marina Rudd has apparently committed suicide.
In the film's denouement, Miss Marple explains the murders that have occurred. Heather Babcock's story was Marina's initial motive. Heather suffered from German measles â a rather harmless disease to most adults, but problematic for a pregnant woman. Heather innocently infected Marina when she met her during the Second World War while Marina was pregnant: she had caused Marina's child to be born with mental retardation. Upon hearing Heather cheerfully tell this story, Marina was overcome with rage and deliberately poisoned her. She then spread the idea that she was the intended victim, concocting the death threats and poisoning her own coffee. Ella, who in fact made phone calls to various suspects from a phone box, accidentally guessed correctly, prompting Marina to murder her. As Marina is now dead, she will not be brought to justice. Jason confesses to Miss Marple that he had put poison in his wife's hot chocolate to save her from being prosecuted; however, the drink has not been touched. Marina is nonetheless found dead, seeming to have poisoned herself.

Question: What did the main suspect, Ella Zielinsky, threaten to do?
Answer: Expose the murderer