Peter Cook's wife, Wendy was nine months pregnant when filming began. Producer and director Bryan Forbes promised them that he would let Peter leave the set as soon as Wendy went into labour. He kept his word, and Peter made it to the hospital just in time for the birth of his daughter, Daisy. Forbes, Dudley Moore, Sir Michael Caine, and Peter Sellers filled his dressing room with flowers and champagne, in celebration of Daisy's birth, when he returned to work.
Bryan Forbes once apologized to Sir Ralph Richardson for keeping him waiting before shooting a scene. Richardson, who had just finished shooting Doctor Zhivago, said "My dear Professor (as he called Forbes) you forget I have just been working for David Lean. Compared to him, you are the speed of light."
When Sir Ralph Richardson was offered the part of Joseph Finsbury, he was finishing work on Doctor Zhivago (1965). Richardson wrote to producer and director Bryan Forbes from the location in Spain, and asked if he could wear the same jacket he had worn as Alexander in Doctor Zhivago (1965). Forbes agreed, and Richardson did so.
The source novel "The Wrong Box" (1889) was the first of three novels that Robert Louis Stevenson co-wrote with his stepson Lloyd Osbourne. The subsequent books were "The Wrecker" (1892) and "The Ebb-Tide" (1894).
This movie was notable for casting British television and radio comedians (Peter Cook, Dudley Moore, Tony Hancock and Peter Sellers) alongside a significant group of the UK's prominent movie stars (John Mills, Ralph Richardson and Michael Caine).