James Cagney did not need boxing training for the film, as in his youth he was an amateur boxer - good enough to be runner-up in the New York State lightweight division.
Danny's newsstand is in the Times Square area - note the signs in the background for the Hotel Astor on Broadway between 44th and 45th Streets (demolished in 1967), Hector's Cafeteria, and the Rialto Theatre at 1481 Broadway (demolished in 2002).
Film debut of Arthur Kennedy, who would go on to become a premier character actor in Hollywood for the next 50 years.
When Danny and Peggy are on the street talking after not seeing each other for awhile, there is a "Koler Cigars" sign in the background. That would put the pair near 235 W. 47th St. between Broadway and 8th Avenue in the Theatre District - near the Hotel Edison and the Barrymore Theatre.
James Cagney's boxing stand-in was Quentin Breese (nicknamed "Baby"), a professional boxer who was ranked as one of the first ten lightweights in the world; he lost only 15 of 100 fights in his career.