Reginald Rose originally wrote the script for 12 Angry Men as a "teleplay," or a play that was meant to be acted out on television, back in 1954. But not everyone realizes that Rose actually based this play on his own experience as a juror a few years before. When asked about it some time later, Rose remembered: "I was on a jury for a manslaughter case, and we got into this terrific, furious, eight-hour argument in the jury room. I was writing one-hour dramas for Studio One then, and I thought, wow, what a setting for a drama" (source).
So it's safe to say that Rose's personal experience strongly influenced the script that would eventually earn him an Oscar nomination for Best Screenplay.