ET's Lauren Zima spoke with Zellweger at the NBC series' premiere in Los Angeles Monday night, where she opened up about the major transformation she underwent to play the Missouri mother turned murderer, Pam Hupp.
"Amazement, I was amazed," Zellweger said of seeing herself as Hupp for the first time. "You might achieve a couple of things, but I had no expectation it was gonna be as remarkable as that."
In the upcoming series, co-starring Josh Duhamel as defense attorney Joel Schwartz, Judy Greer as prosecutor Leah Askey and Katy Mixon as Betsy Faria, Zellweger plays Pam, whose deeply diabolical and deadly scheme was exposed by a chain of events following the overturning of Russ Faria’s conviction in the 2011 murder of his wife.
In addition to transforming her face for the role, Zellweger also donned a body prosthetic, in an effort to make her appearance as Hupp as "accurate" as possible.
"The idea is to be accurate, the idea is accuracy," the Bridget Jones's Diary star said of why she chose to wear the sometimes-daunting prosthetic. "I think, especially in the case of telling this story, it was really important to as closely resemble Pam Hupp as we possibly could, because she seems so familiar, she seems like someone that we recognize, and we know."
"In order for you to better understand how possible it might be that people would project onto her who they are sure that she might be or what kind of person she might, it just seemed really important that we got as close to that as we could," she continued.
Zellweger was in the makeup chair for hours a day for the series, which filmed in New Orleans, a city which has its fair share of hot and humid weather.
"It was hot. It was really hot," she said with a laugh. "But it's amazing to watch. Arian, who builds these things and applies them every day, he creates them by hand and paints them down to the last freckle. His precision is remarkable. It's part of the adventure, watching that happen every day, is pretty cool."
Though Zellweger was filming in a town known for its live jazz music and exciting nightlife, when she did have a break from filming, she rarely enjoyed all that the Southern city had to offer.
"Oh no, no, no," Zellweger said about using her time away from set to unwind. "No, 'cause you have to be ready for tomorrow, so you have to do your homework when you get home."
She continued, "You just have your list of things that you know you need to do, so you just do it. You just wanna be responsible. Everybody's working so hard, you don't want to be the jerk who shows up the next day not ready, no thanks."
You can see the hardworking actress when The Thing About Pam premieres March 8 at 10:00 p.m. PT on NBC.