Baker's inability to throw the ball back to the pitcher was based on former New York Mets catcher Mackey Sasser. Sasser was very successful at throwing out runners attempting to steal second, but had a mental block causing him to flinch several times when trying to throw the ball back to the pitcher. Many baserunners took advantage of this affliction and stole bases while he was triple pumping his throw, and/or lollipopping the throw back.
While Camden Yards doubled as Cleveland Municipal Stadium, scenes depicting Indians road games were filmed mostly at nearby Baltimore Memorial Stadium.
The character Isuro Tanaka was something of an anomaly of that time. At the time of the film's release, there were no Japanese players in the Major Leagues, and only one Japanese player had ever played (briefly) for a ML team. The influx of Japanese players in US Major League baseball didn't begin until a few years after the film's release.
Kevin Hickey, who plays Indians pitcher Schoup, made his major league debut in 1981 as a pitcher for the Chicago White Sox, the very team the fictional pitcher Schoup plays against in the film.