To prepare for his lead role as Mick O'Brien, actor Sean Penn let his hair grow long, had a tattoo inked on his arm, dressed in street gang garb, went on night patrol with the Chicago Police Gang Crimes Unit and almost got himself arrested at one point whilst doing all this.
To play youth thug Mick o'Brien, actor Sean Penn wanted to have his perfect teeth filed down and capped with ugly tops and have "maybe a few cracked ones" but Penn's mother, a dentist's daughter, intervened, ruling that "ruining perfectly good teeth is definitely going too far!".
To prepare for the movie, Rick Rosenthal and Sean Penn went out riding with the Chicago Police Dept. Gang Squad Unit. On the second night, Penn was mistaken for a real criminal and was assaulted by one of the officers.
During the scene where Sean Penn's character first enters the reform school and walks down the line in between the prisoners, Penn encouraged the actors to spit in his face and throw things at him. On the other hand, Esai Morales insisted that the actors who played the prisoners brush their teeth and use mouthwash before they spit in his face during his character's similar scene.
Sean Penn insisted on being called the name of his character, Mick O'Brien, during production of the movie. Moreover, when Penn spent a week's filming at the St. Charles maximum-security youth reformatory, his official name-tag identification read "Mick O'Brien".
The passage written on the board in the classroom is from Jack London's 'Call of the Wild'. Presumably, this is also the novel O'Brien and Horowitz read and discuss while sitting in their room during a later scene.
The name of the youth jail was the "Rainford Juvenile Correctional Facility". Established in 1919, it's motto was "Justice with Mercy". The name of the other prison facility, the gaol for adults, was the "Illinois State Penitentiary".
Sean Penn once said of this film around the time of its first release: "This movie is a lot about drive, about frustrated drive and we all experience that. The kids in this movie feel they have very few choices - and they're unaware of those additional options they may have. I read the script and felt it was real. I went out into the areas of Chicago where we were going to shoot and there was stuff right out of our script, and even rougher. In a funny way I think people who see this film will be relieved to see that the rest of the world isn't like that. I want to see more films about the way the world 'really' is. With those so-called 'escapist' or 'pure entertainment' movies, you come out and the world seems so dull you want to go back inside. But I think reality should be more fun than fantasy. I think the real world's the most exciting of all, and that whatever is wrong with it can be fixed. I'm an opportunist".
One of only four feature films featuring actor Eric Gurry who played Sean Penn's little cell-mate side-kick Horowitz. This was Gurry's second feature film after just being introduced to audiences the year before playing one of Al Pacino's children in Author! Author! (1982).
In 1995, around twelve years after this film first was released, actor Sean Penn would portray a prison inmate again in Dead Man Walking (1995). This was the same year when the other Bad Boys (1995) movie came out.