Irvin Winkler (no relation to the Fonz) is a producer/director, who produced every Rocky movie, TheWolf of Wall Street, and directed the movie that got everything about the Internet wrong: The Net with Sandra Bullock (1995). Along with producing partner Robert Chartoff, Winkler purchased Rocky and brought it to United Artists, who greenlit the film with a budget of only $1 million.
Despite the fact that they were enthusiastic about the script, they didn't want Stallone playing the role of Rocky. Ryan O'Neal, Robert Redford, and even Burt Reynolds were all big-name actors considered for the role. Can you imagine Smokey (or was he the Bandit) as Rocky Balboa?
But after some begging by Stallone—and the fact that he was way cheaper—they agreed to cast him in the iconic part. (Source)
Because of the low budget, the film was made at a breakneck pace of only 28 days, and filmed on the run throughout the streets of Philadelphia. Many of the film's iconic scenes, like a stranger tossing an orange to Rocky, were spontaneous… and the film wouldn't be the same without them. (Source)