Nope, it unfolded on camera – in front of an entire film crew – while shooting The Last Song.
"We turned up on set the very first day and originally it was just a scene where we were running through the water and splashing and having fun," Hemsworth told Ryan Seacrest on his KIIS-FM radio show. "And halfway through the scene, the director, Julie Anne [Robinson], she yelled 'Kiss!' And we got thrown into a kiss on the boat."
But the Australian actor wasn't too disappointed about how it happened. "It was good, 'cause, you know, [you] get the first one out of the way, and it was fine," Hemsworth, 20, said.
Plus, he said, it was "hard not to get close" to his costar while filming in Tybee Island, Ga. – a remote location, especially compared to Hollywood. "We were filming together every day, and hanging out every day," he said. "We really didn’t have any other friends down there, so we just hung out with each other."
It also helped that Cyrus's mother, Tish, whom Hemsworth described as "the boss," gave her stamp of approval on the budding relationship. "I got along well with her too, so I don't think she minded," Hemsworth said.
Getting SeriousDad Billy Ray, meanwhile, had another pressing question for Hemsworth, which Seacrest relayed: "How serious is it gonna get?"
Hemsworth's reply: "It's pretty serious."
Recently, the couple stepped out on the red carpet for the Los Angeles premiere of The Last Song, which arrives in theaters Wednesday. But Hemsworth said he's still getting used to the media spotlight – not to mention the added attention that comes with his high-profile girlfriend.
"It's all very new to me," he said. "I've never experienced anything quite as big as this."
Get a sneak peek of this week's issue – Sandra Bullock's divorce dilemma, new Jolie-Pitt twin photos and more – which is on newsstands Friday