Harry Melling plays a person with magic in this episode, the very opposite of one of his most famous roles, Dudley Dursley in Harry Potter, the son of parents who absolutely despise magic. However, there is a headcannon that Dudley had magical kids and he and Harry made up.
Colin Morgan (Merlin) had previously worked with Harry Melling (Gilli)'s uncle, David Troughton in the Doctor Who episode Midnight (2008). They played Jethro and Professor Hobbes respectively.
Anthony Head (Uther Pendragon), Angel Coulby (Gwen - who does not appear in this episode) and Harry Melling (Gilli)'s cousin, Sam Troughton have appeared in the TV mini-series Dancing on the Edge (2013). They played Donaldson, Jessie and Prince of Wales respectively. Also starring in the show was Janet Montgomery, who in later episodes of Merlin played the character of Princess Mithian.