The Sex and the City star stepped in to play feminist icon Gloria Steinem after Moore dropped out of the project in January (12) to seek treatment for exhaustion.
Seyfried, who plays X-rated actress-turned-anti-porn activist Linda Lovelace in the film, is taking full credit for the last-minute casting coup.
She tells U.K. talk show host Jonathan Ross, "(Moore) was ill so she couldn't show up. When we found out she wasn't coming to work, we were just like, s**t! But the fact is, she couldn't come so we had to find somebody else and so I emailed Sarah Jessica Parker myself.
"I feel like (casting is important) when it comes from a personal point, everything's so business-y and you have to find some kind of connection in this business if you want to get what you want. I really, really love her so I just emailed her. It worked - something happened - she came, she showed up a couple days later. And she is bada** in this role."