The usually skinny star packed on 20 pounds (nine kilograms) for her latest role - but her transformation was a difficult one to watch for Paltrow's fitness trainer Tracy Anderson.
She tells People.com, "They wanted her to eat a lot of fried chicken. It was frustrating for me, but they wanted her to gain (weight).
"When I trained her for the first Iron Man movie, I took 30 pounds (13.6 kilograms) off her. And she's been consistent ever since."
Paltrow managed to shed the extra weight soon after filming wrapped, and she's set to show off her toned figure when she takes the stage to make her live singing debut at the Country Music Association Awards on Wednesday (10Nov10).