In the novel 4.50 from Paddington, Dorothy Cartwright is an acquaintance of Jane Marple. Miss Marple said to Elspeth McGillicuddy that if Dorothy had said she’d seen a murder, it was not as likely to be true as if Elspeth said it.
This was because Dorothy always had a good story, and quite often believed it herself, and there was usually a kind of basis of truth, but no more. On the other hand, Elspeth was the kind of woman who found it very hard to make herself believe that anything extraordinary could happen to her. So, when Elspeth said that she had seen a murder, Miss Marple believed that it was true.