Garner tackled Spears first, perfectly capturing the pop star's iconic movements and the timbre of her voice. She next explained that although Stefani is in "the same family" of impressions as Spears, the former's voice is grittier and her movements are more emotional.
"Your body turned into Gwen Stefani right now. And your face!" Fallon exclaimed after Garner's impression. "That's perfect! You're amazing!"
During her appearance on the show, Garner also spoke about the night she won an Emmy for her role in Netflix's Ozark, an award that she's nominated for again this year.
"To be honest, [I only remember] bits and pieces," Garner said of the night she won the Emmy. "It's like an out-of-body experience... It was kind of a weird thing. And then when you go onstage, you kind of black out."
Watch the video for ET's interview with the actress following her Emmy win.