In the novel They do it with Mirrors, Ernie Gregg was one of the delinquents who was active in the theatre work, and was killed for boasting to Gina about what he says he saw the night of the murder. He likely made it up.
Ernie had played Feste in Stephen Restarick's production of Twelfth Night, and both Restarick brothers felt that he had some talent in acting. Stephen also said that Ernie was good with his hands, and did a lot of carpentry work in the theatre.
Ernie claimed that he had been out on the night of the murder, and had seen something, but he would not say what it was. Miss Marple thought that he was likely just boasting, similar to Johnnie Backhouse from St Mary Mead.
Ernie was missing from roll call that night, and Dr Maverick and some of the staff searched for him. They questioned another boy, Arthur Jenkins, who told them that Ernie had not been outside on the night of the murder, and had never gotten out at night at all. He said that Ernie boasted that he could pick locks, but actually could not.
In Murder with Mirrors (1985), Ernie Gregg and the entire subplot is not featured.
In the BBC 1991 adaptation of the novel which formed part of the Miss Marple series, Ernie Gregg is played by Matthew Cottle. Here the protrayal is close to the novel. He also takes part in the Stonygates amateur dramatics and knows Stephen Restarick. In one production Ernie played the Artful Dodger. Stephen Restarick told Alexis Restarick that Ernie told Gina that he had seen something but that he had a reputation for lying. The brothers decide to test him out before deciding if his testimony is worth repeating to the police. Accordingly, Alexis dresses up as a detective and arranges to meet Ernie in the theatre. Just as the meeting is about to take place, a gantry of stage lights falls, killing Ernie and injuring Alexis.
In the ITV 2009 adaptation of the novel as part of their Agatha Christie's Marple series, Ernie is not featured. There is another character, a delinquent named Whitstable Ernest. Despite the similarity in the name, he plays a different role.