While making The Charge of the Light Brigade (1936) at Lone Pine, California, he befriended Errol Flynn, whose acquaintance he had made in England when both were under contract to Warner Bros. at Teddington Studios. Since that film, in which Knowles played the part of Capt. Perry Vickers, the brother of Flynn's Maj. Geoffrey Vickers, he was cast more frequently as straitlaced characters alongside Flynn's flamboyant ones, notably as Will Scarlet in The Adventures of Robin Hood (1938). Both actors starred as well in Four's a Crowd, also in 1938. More than two decades after Flynn's death, biographer Charles Higham accused Flynn of having been a fascist sympathizer and Nazi spy. Knowles, who had served in World War II as a flying instructor in the Royal Canadian Air Force, came to Flynn's defense, writing Rebuttal for a Friend as an epilogue to Tony Thomas' Errol Flynn: The Spy Who Never Was (Citadel Press, 1990) ISBN 0-8065-1180-X.