Run, Forrest, Run
“Run, Forrest! Run!” There’s another line from the movie that is still quoted until today. Funnily enough, Forrest was literally running for large portions of the movie. Tom Hanks, however, wasn’t much of a runner. But his brother was. Tom’s brother, Jim Hanks, was actually a runner. And guess what? He happens to look a lot like Tom, too.
Source: Uproxx
So it was a simple and easy solution for the filmmakers. They swapped Tom for Jim whenever a scene called for Forrest to run long-distance.
James Mathew Hanks is not just an actor, but a voice actor, director, producer, cinematographer, and camera operator. He has minor roles in films and guest appearances on TV, and he often substitutes doing voice work for Tom Hanks.