The screenplay of the film stayed as close to the truth as was convenient, with the remainder being fiction. For example, the Irish spy in the film is a complete fabrication. Ewen Montagu declared that he was happy with the fictitious incidents which, although they did not happen, might have happened. During filming, Montagu has a cameo role, that of a Royal Air Force air vice-marshal who has doubts about the feasibility of the proposed plan. It was described as a "surreal" moment when the real Montagu addressed his fictional persona, played by Webb.