After apprehensively taking the small bottle, Styles gently blew into it and sealed it back up quickly.
Immediately upon closing the bottle, Fallon clutched his chest and fell to the floor, pretending to die. Without missing a beat, the 23-year-old performer walked from the couch, around Fallon's motionless body, and took a seat in the host's chair.
"Hey! We've got a great show for you tonight! Harry Styles is here!" he yelled, living out his true dream of hosting the Tonight Show, if even for a brief moment.
"I'm such a fan of his, and I'm such a fan of so many people working on this film," Styles marveled. "I felt so lucky to be there, to be honest."
Dunkirk tells the true story of one of the most harrowing battles of World War II, in which Allied soldiers from Britain, Belgium, Canada, and France were evacuated from the harbor in Dunkirk, France, during Operation Dynamo, as Nazi forces surrounded them and moved in to attack.
During the evacuation, civilian vessels were called on to support the evacuation, and many families risked life and limb to help in the dangerous military operation.
"It's kind of the opposite of what a traditional movie [about] war is," Styles reflected. "It's not about us being heroes and going and saving the day. It's just about community heroism and about how these small acts from civilians and the normal people coming to save us makes this thing. It's very much about the spirit of people and how, when people come together, you can do incredible things."
"When I cast Heath Ledger as the Joker [in The Dark Knight], it raised a lot of eyebrows and caused a lot of comment," he added. "I have to trust my instincts, and Harry was perfect for this part." Check out the video below to hear more.