One of his earliest film collaborations was productions was with Frank Launder creating the Charters and Caldicott characters in Hitchcock’s The Lady Vanishes film in 1938 which were resurrected in the 1941 film Night Train To Munich 1941 directed by Carol Reed. The two writers then made their co-directional debut on Millions Like Us in 1943 which also featured the Charters and Caldicott characters played by Basil Radford and Naunton Wayne.
Other films that Gilliat worked on include The Man Who Changed His Mind 1936 starring Boris Karloff, as a writer on Will Hay’s Ask a Policeman 1938, Waterloo Road 1944, Two Thousand Women 1944, I See A Dark Stranger 1946, Four of the St Trinian's films, The Constant Husband 1955, The Green Man 1956 and The Bridal Path in 1959.