Miss Johnson was about fifty years old, and was "rather mannish in appearance, with iron-grey hair cropped short". She had an "abrupt, pleasant voice, rather deep in tone". Her face was ugly and rugged, "with an almost laughably turned-up nose which she was in the habit of rubbing irritably when anything troubled or perplexed her". She was a native of Yorkshire.
Miss Johnson reminded Nurse Leatheran of a matron she had had in her probationer days. She was capab;e, practical, and intelligent. Nurse Leatheran also observed that Miss Johnson had a distinct hero worship for Dr Leidner, whom she had known for seven years. She knew every site he had dug, and the result of the dig. She considered him the finest field archaeologist living. She also admired him for being so simple and light-hearted.
Miss Johnson was of the opinion that if an archaeologist's wife was not interested, it would be wiser for her not to accompany the expedition. She felt that Louise Leidner's presence unsettled things, and that Dr Leidner's mind should be on the work, not on his wife and her fears.
At the time of Louise's murder, Miss Johnson had been in the living room, taking impressions of some cylinder seals on plasticine. All the windows of the living room were open. She thought she had heard a cry. After the discovery of the murder, she got the idea that it was Louise who had given the cry, and was rather unhappy because she thought that if she had jumped up and gone to Louise's room, she might have been able to do something.
Later in the novel, Nurse Leatheran found Miss Johnson alone in the office, crying "as though her heart would break". Miss Johnson said that she had thought it was best to do something, and had gone into the office to tidy things up, when the realisation of how awful the situation was came over her suddenly. Nurse Leatheran calmed her down, and got her to go to bed.
Nurse Leatheran noticed a crumpled piece of paper on the floor of Miss Johnson's bedroom, and smoothed it out to see if it could safely be thrown away. Miss Johnson snatched it from her, and burned it, but Nurse Leatheran was able to observe that the handwriting was the same as the anonymous letters Louise had received.
Poirot later asked Miss Johnson for her opinion on the anonymous letters. Miss Johnson believed that someone in America had a spite against Louise, and was trying to frighten or annoy her. This showed that she knew about the anonymous letters, although they had not been mentioned in the presence of the expedition staff.
Poirot also asked Miss Johnson if there was an attraction between Sheila Reilly and one of the young men at the expedition. She said that both Bill Coleman and David Emmott danced attendance on Sheila, and that Sheila often came up to the dig. Miss Johnson had seen Sheila riding up to the dig on the afternoon of the murder. She asked if this had any bearing on the crime, and Poirot explained that he was always interested in the love affairs of young people. Miss Johnson sighed, and said that it was nice when the course of true love ran smooth.
Later in the novel, Nurse Leatheran found Miss Johnson on the roof of the expedition house, staring straight in front of her, as if she had seen something she could not believe. Miss Johnson said that she had seen how someone could get into the expedition house from the outside, and no one would ever guess. She did not explain what she had seen, as she said she had to think it out first.
That night, Nurse Leatheran found Miss Johnson in agony, having drunk corrosive acid. They treated her with a strong solution of carbonate of soda, followed by olive oil, and Nurse Leatheran gave her a hypodermic of morphine sulphate. However, they were unable to save her. Before she died, Miss Johnson mentioned "the window".
It was later found that Miss Johnson had drunk a strong solution of hydrocholoric acid, which had been taken from the laboratory at the expedition house. A heavy quern was also found under her bed, which had a dark stain and a fragment of something that looked like hair on it. This was thought to have been the weapon used to kill Louise Leidner.
Miss Johnson's death was initially thought to be suicide. However, it is later revealed that she was murdered, because she had realised that Louise was killed through the window of her bedroom, which was open at the time. The murderer substituted a glass of hydrochloric acid for the glass of water that Miss Johnson always took to bed with her, and also planted the quern under her bed. Miss Johnson drank the acid while she was still half-asleep, before realising what it was.
At the end of the novel, Poirot explains that Miss Johnson must have come upon an unfinished draft of one of the anonymous letters when she was tidying up the office. She could not understand why Dr Leidner would deliberately terrorise his wife, and it upset her badly. This was why she was crying in the office when Nurse Leatheran found her.
Poirot also explains that his experiments with sounds in Louise's and Father Lavigny's rooms were not lost on Miss Johnson, and that she realised that if it was Louise's cry that she had heard on the day of the murder, her bedroom window must have been open, and not closed. At that time, that conveyed nothing vital to her, but she remembered it.
One evening, when Miss Johnson was on the roof, she realised that Louise had been killed from the roof, through the open window of her bedroom. However, when Nurse Leatheran found her, and asked what the matter was, her old affection for Dr Leidner reasserted itself. Miss Johnson deliberately looked in the opposite direction, and said that she had seen how someone could get in from the outside without anyone knowing. Dr Leidner realised that she knew the truth, because she was not the kind of person to conceal her horror and distress from him, and so he killed her.
In the ITV 2001 TV adapatation of the novel, the part of Anne Johnson is played by Dinah Stabb. The portrayal is fairly faithful to the original novel. Like Richard Carey, she is a long-time archaeological colleague of Eric Leidner and had been one of his partners at the dig of Tel Yarimjah from the time before Leidner married Louise. She described those early years as filled with fun as they worked so well together as a team. She felt that the arrival of Louise spoilt everything. There is a slight difference in the critical roof-top scene with Poirot. In this adaptation, when Poirot met her on the roof-top, she told him that she had just discovered one means by which someone could enter the courtyard and no one would know about it. Poirot later surmised that she wanted to say something else but Eric Leidner had just turned up on the roof-top and was standing behind Poirot. Anne was afraid that Leidner would overhear and so had to quickly change her message. In the original, Leidner was not on the roof and the implication is that Anne there was being oblique because she loved Leidner and could not bring herself to betray him. In this adaptation Leidner had heard enough on the roof to become worried and so poisoned her with sulphuric acid (not hydrochloric in the original). Her dying words to Poirot was "the window" and this was enough to help Poirot work out how Louise had been killed.