The actress, who welcomed baby James with Ryan Reynolds this past December, got devilishly into the spirit of the hugely popular YouTube game. The rules are simple: Each person takes turns blurting out a random word until the other one stumbles, blanks, or repeats something that's already been said. The winner of that turn then gets to apply a long piece of clear tape to their opponent's face.
Lively got off to a, um, lively start when she made Fallon sputter with her word "stingy," and taped The Tonight Show host's eyebrows up. "You look like Leonard Nimoy," she told him.
She continued with the Star Trek theme when Fallon messed up the next round as well, and gave him a pair of bulbous William Shatner cheeks. Fallon eventually lost the game on the third go-around—watch the video above to see his final facial alteration!