In a new interview, Tom says that he and director ROBERT ZEMECKIS covered the cost of some scenes that the studio didn't want to pay for.
One of them was the part where Forrest runs across the country. Paramount said it was too expensive to film, but Tom and Robert really wanted it.
There was another, unspecified scene where Paramount didn't want to spring for the insurance, so they stepped up again.
In return for putting their own money in, Hanks and Zemeckis asked Paramount to give them a bigger share of the profits. And because of that, Tom ended up making $65 million off"Forrest Gump".
(You can listen to the interview here.)