So what was it like singing to the gladiator himself?
"Surreal!" Watson told MTV News. "I've never really thought to sing in a movie before, except that I'm crying so much that you don't really get to hear much of the song."
The emotional moment occurs aboard the ark, when (spoilers) an apocalyptic storm has wiped out the majority of the world's population, leaving only Noah (Crowe) and his family as survivors.
Watson plays Ila, Noah's adoptive daughter and wife to his eldest son, Shem (Douglas Booth). It's one of the many unexpected moments in a film already packed with stone angels, "Lord of the Rings" style battles, and a massive, CG menagerie of birds, beasts and bugs.
Since co-star Russell Crowe is something of a singer himself — he played Javert in the 2012 version of "Les Miserables" and fronted his own band, 30 Odd Foot of Grunts — did he have any notes for his co-star during her big moment?
"You know he didn't," Watson says. "And I think that was a compliment."
"Noah" will be in theaters on March 28.