Grandfather (John McCartney) was a fictional character from the movie, A Hard Day's Night played by actor Wilfred Brambell. In the movie, he was the grandfather of Paul McCartney, bass guitarist for The Beatles. Paul brought him because his mother thought the trip would help him, because he was "nursing a broken heart." While riding the train, the question "Who's that little old man?" is asked several times. Shortly after Paul's response that the old man was his grandfather, George comments, "That's not your grandfather. I've seen your grandfather; he lives in your house." Paul answers, "That's my other grandfather, but he's my grandfather as well." This is repeated when, after saying he was Paul's grandfather, Norm (the Beatles' manager in the film) says, "Oh, I thought that--" stopped by John Lennon, saying, "No, that's his other one."
The character of Paul's grandfather is described in the film as a "villain" and a "real mixer" and acts something as the antagonist of the film. He sabotages the boys' attempts to flirt on the train, plagiarizes their signatures on photographs, and generally causes minor havoc throughout the story.
After Ringo is recruited to look after the old man during some down time before the Beatles' scheduled television performance, he is told that he shouldn't be reading a book, but living until it's too late. Ringo takes this crazy advice and leaves the television station. This results in the main conflict of the film as the managers, producers, and other three Beatles go on a search to find the drummer in time to perform.
Throughout the film several characters note that Grandfather is "very clean", this a reference to Brambell's other famous role as Albert Steptoe on the British sitcom, Steptoe and Son, who was often called "a dirty old man" by his son, Harold.