The NBC drama, which stars Josh Lucas and Molly Parker, has received mixed reviews from the US press.
"It's tricky because we, the actors, took the project off of one script," she told New York Magazine. "That's part of the intrigue of taking this job. You have to act whatever you're given."
Lewis, who plays legal secretary Tammy Hemphill, went on to describe "bad reviews" as "weird" and "irrelevant".
"I probably would be sad if it was my project, like if I developed it," she said. "But I'm not. So I'm just trying to be good at my part."
The Firm has also struggled in the ratings, with the latest episode attracting just 4.23m viewers on Thursday night.
The series continues on Thursdays at 10/9c on NBC.
> The Firm adds Shaun Majumder to cast
Watch a clip from The Firm below: