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The 64-year-old actor said he was asked to fund some of the most powerful scenes of the blockbuster, which ended up making him about $65 million (£51m).
Oscar-winner Hanks said both he and director Robert Zemeckis contributed financially when the production company Paramount wouldn't splash out on the 1994 movie.
Forrest's classic run across the country scene was one segment that could have looked very different - when Zemeckis told Paramount that it was important, Hanks was asked to chip in.
Hanks told In Depth With Graham Bensinger: "And he said, 'Well, this run is going to cost X amount of dollars.' And it wasn't cheap. And I said, 'OK'.
"He said, 'You and I are going to split that amount, and we're going to give it back [to Paramount]. We'll give you the money back, but you guys [Paramount] are going to have to share the profits a little bit more.' Which the studio said, 'Fabulous, great. OK.' And it was good for us, too."
Another time the same thing happened, Hanks didn't say exactly which scene it was, but explained what happened.
He explained: "They said, 'The weather is such that we can't get the insurance coverage on it,' the studio said, 'So you guys can't shoot.' And Bob and I said, 'We'll cover the insurance.' And we did."
It was all well worth the investment though, with Hanks earning about $65 million from the film, which is now a nailed-on classic.
Although he's an incredibly accomplished actor, he mentioned that he didn't have an easy time with the character at the start of the film. He even said the footage from the first three days was scrapped in the end.
Hanks said: "Bob said, 'Look, I know what you are trying to do. I know how nervous you are and how self-conscious this can be before we get into the groove. But we're not going to use any of these first three days because I don't think you've got it. You haven't got the character.'
"And I said, 'I don't. I don't. You're right.' And he just said, 'Don't try so hard.' And from that, everything settled down in a moment's notice."
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