Hayley Atwell surprised her Howards End castmates by playing pranks on them on set.
The British actress, 35, plays intellectual socialite Margaret Schlegel in a new TV adaptation of E.M. Forster's 1910 novel, which she filmed earlier this year (17).
Her castmates in the period drama outed her as a serial prankster, telling WENN/Cover Media that she targeted her co-star Matthew Macfadyen, 43, in particular.
And the actress confessed that she had turned Matthew's trailer into an infant's nursery to mock the behavior of his character, Margaret's love interest Henry Wilcox.
She explained, "Matthew Macfadyen and I had this ongoing running joke about the fact that maybe Margaret Schlegel, my character changes him by the end of the piece - and was it always a hidden attempt to just infantilise him and make him a baby. We had this idea that he was just sat at the end of the film eating (the baby food) Rusks in a nappy.
"So on his last day filming I went into his trailer and I put a training potty over his toilet, a nappy in his trousers, a dummy in his pocket, nappy rash cream in his make-up station and then every time he asked for a cup of tea on set I had the runners give it to him in a sippy cup with a bib. So he got to spend the day as a big baby, which was really fun. "
Australian actress Philippa Coulthard, who plays Margaret's sister Helen in the TV mini-series, called the Agent Carter star "the queen of pranks" and Hayley explained her love of practical jokes by saying, "We just like making each other laugh - it makes the day go quicker and makes things much more enjoyable."
Another co-star, Bessie Carter said Hayley's high jinks had kept the cast "relaxed" on set and that they had become firm friends who love taking on escape rooms - puzzles where groups of friends have to break free from a locked room.
After revealing that she and the star had formed a WhatsApp group to discuss their hobby she gushed, "Hayley is the queen of escape rooms."
Howards End debuts on British channel BBC One on 12 November (17).