- In the film, Gwyneth wants to break through to become a fat man weighing more than 300 kilograms. For this reason, she had to wear a "fat costume" made of artificial fat to achieve the effect. This suit is very uncomfortable and even breathless after putting it on. She also needs to stick the plastic mask on her face, each time it takes 3 hours to stick, and it takes another 1 hour to remove makeup.
- Gwyneth once took to the streets as a fat girl "Rosemary" during the filming, and went to hang out in the hotel lobby, hoping to learn about the daily life of a fat man.
- The film is the first time that Jack Black has played the leading role, and he has performed weight loss specifically for the role of Hal.
- There is a scene where Rosemary jumps into the swimming pool and splashes huge water splashes. When shooting, two very thick hoses were used to spray water jets from both sides of Rosemary to simulate huge splashes.
Shallow Hal behind the scenes gags
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[after Mauricio broke Hal's spell]
Hal: Let me ask you something. Who is the all-time love of your life?
Mauricio: [ponders] Wonder Woman.
Hal: Okay... let's say Wonder Woman falls in love with you. And everyone else in the world didn't find her attractive.
Mauricio: It wouldn't matter. Because I know they'd be wrong.
Hal: See! That's what I had with Rosemary! I saw a knockout, I don't care what anybody else saw!
Mauricio: You're right. I guess I really did screw you, huh?
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Mauricio: Shallow Hal wants a gal.