The shots of Billy Chapel's parents throughout the movie are Kevin Costner's actual parents.
According to Kevin Costner, Yankees owner George Steinbrenner was becoming agitated that the Yankees would be seen losing in a movie filmed on location at Yankee Stadium. Costner said he was able to calm Steinbrenner down by explaining that Billy Chapel, and not necessarily the game itself, was the movie's major focus. Costner also explained that the game would mean nothing to the Yankees, and that it wasn't played as a "Winner Take All" scenario. Costner convinced Steinbrenner that in the movie's universe, the Yankees would clinch the AL East division the next day, keeping alive their World Series chances. Costner said that he later received a Yankees 1999 World Series ring as a thank-you gift from Steinbrenner.
All four umpires were played by real Major League umpires.
Dave Eiland (the only major league player ever to give up a homerun to the first hitter he faced, and hit a homerun in his first plate appearance), who played a relief pitcher in the bullpen and celebration scene, was also Kevin Costner's pitching double in some long shots. He later went on to serve as pitching coach for the Kansas City Royals and New York Mets.
Many of the Yankees players were played by Yankee minor leaguers.