Many facts were changed or ignored to add to the feel of the movie. For example, the real George M. Cohan was married twice, and although his second wife's middle name was Mary, she went by her first name, Agnes. In fact, the movie deviated from the truth to such a degree that Cohan's daughter Georgette commented, "That's the kind of life Daddy would have liked to have lived."
Despite failing health, the real George M. Cohan acted briefly as a consultant on the film. He lived long enough to see the finished result and approved wholeheartedly of James Cagney's depiction of himself.
According to his biography the rather stiff-legged dancing style used by James Cagney in this movie is not his own. He copied George M. Cohan's style to make the film more accurate.
Walking down the stairs at the White House, James Cagney goes into a tap dance. According to TCM, that was completely ad-libbed.