Josh Hartnettwasnt prepared for the bombshell adviceMatt Damondropped on him while filmingOppenheimer.

He gave me a lot of good advice.

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Split frame of Josh Hartnett and Matt Damon at red carpet events

Josh Hartnett and Matt Damon.Getty(2)

And he kept telling me, like, over the course of the production.

Hartnett could only take Damons delayed advice in stride.

He teased, I was like, Thanks, Matt.

Thanks for telling me this now.

Ive already gained it.

Damon is no stranger to undergoing weight transformations for a role.

And I wanted to venture to create more of that sort of underdog feeling.

Watch Hartnett recall Damons advice in the clip above.