and 50 cuts were made in the first 10 minutes of the pilot episode to quickly introduce the main characters. Filming was usually done with long lens cameras, which proved difficult for the crew as many filming location were short on space. The scenes where Tom reveals he is a spy to Ellie in "Traitor's Gate" were filmed using hand-held cameras, the first time they were used in the series. By the time the crew filmed the series finale, hand-held cameras were mostly used. The cameras allowed for more intimate filming as opposed to shoulder-operated cameras. The downside to the hand-held cameras was that they can only shoot four minutes worth of footage from each film stock. In addition, the operators at first hated using the cameras. The episodes of the first series were not filmed in the order they were broadcast; "Traitor's Gate" was the third episode to be filmed, while "One Last Dance" was the fourth. While filming "One Last Dance" and "The Rose Bed Memoirs", Macfadyen had a bloodshot left eye. To cover it, sometimes Macfadyen had to hide the left side of his face from the camera. At other times, the actor wore sunglasses. The series made use of real firearms that were decommissioned from service. Certain cast members, including Oyelowo, handled firearms at some point in the series. To maintain gun safety, two weapons experts were on hand to train the actors during rehearsal. The producers chose the exterior and some interior of Freemasons' Hall in Holborn as the stand-in for Thames House, the real-life headquarters of MI5, as they felt both buildings shared a similar architecture. However the Grid set, the offices of the protagonists, was filmed in a closed medical school in Kensington. Sometime after filming concluded for the first series, the building was demolished. Several other locales around London were used as filming locations. The scenes leading up to Mary's apprehension in "Thou Shalt Not Kill" were filmed at Borough Market in Southwark. The same episode made use of Covent Garden. "One Last Dance" had scenes filmed in Roehampton and New Zealand House. In the latter's case, the actors were filmed on the top floors, where the wind forced the actors to raise their voices so the sound unit could hear them. Scenes were filmed in Hampstead Heath in "Traitor's Gate", while one scene was filmed in Trafalgar Square and at London