"You have to promise you won't fall in love with me," Fallon read, while struggling to hold in laughter. After Moore correctly says that the line is from the 2002 film, the late-night host made a self-deprecating remark.
"This is why my romantic comedy career went south fast," he quipped.
Moore ended up scoring 100 percent on the line identification test! She remembered that "I thought I saw some something in you, something good, but I was very wrong" was from the teen movie, while the line "For the past 16 years I have put everything and everyone ahead of myself" was said on the NBC drama.
The actress-singer even scored some bonus points with the last line, "I can't go back to who I was before I met you." She noted that Jack Pearson (Milo Ventimiligia) was the one to say it on the series.
In regard to the beloved show, in which Moore plays Rebecca Pearson, she couldn't be more grateful. "I feel like because I'm the only cast member that sort of gets to play with everybody," she told Fallon of This Is Us. "I'm in all the eras of the show."
One thing that goes with playing her character around age 60 is a lot of impressive prosthetic work. While the transition to present-day Rebecca may appear seamless on screen, Moore revealed that it's much harder than it looks to keep the heavy makeup on -- and her co-stars are to blame!
"I have prosthetics on and everyone's trying to lighten the mood by laughing... But when I'm with that side of the family it's tough because Sterling [K. Brown] and Justin [Hartley] and Chris Sullivan are all such jokesters. They're all laughing, and I'm not supposed to move in my prosthetics too much. The more you move and talk and laugh, it loosens them up," she explained. "We realized early on in the show that tears are sort of the perfect solvent for removing the prosthetics."
She continued, "I'm like, 'You guys, we've got to come up with a different glue or something.' Because my face would start melting like halfway through the day and they're, like, trying to spackle me together and keep me going."
ET caught up with Moore in July where she called the series' upcoming third season -- which premieres Tuesday night on NBC -- "our best season yet."
"It's such a fun time because it's kind of been on the record that our story lines like for Jack and Rebecca, at least one of them we're really exploring heavily this season, is when they first meet," Moore said. "It's so much more fun to be happy and in '70s hair and makeup. All of that combined and young love is so much more fun to portray than the heaviness and the dark cloud that sort of hung over season two. I feel like it's some of our best stuff yet."