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Paranoia, Amber Heard, Liam Hemsworth

Relativity Media

Amber Heard couldn't resist pulling an epic prank on Miley Cyrus' man, Liam Hemsworth, on the set of their new thriller, Paranoia.

At the film's L.A. premiere Thursday night, Johnny Depp's gal-pal spilled to reporters what precipitated the hilarious hoax, which took place during the twosome's steamy sex scenes when Hemsworth went to grab what he thought was Heard's arm only to wind up holding a giant dildo.

"It's not a little prank. It was a big prank," Amber said with a wink.

Amber Heard

Jordan Strauss/Invision/AP

When asked how she got the director, Robert Luketic, and crew involved, Heard happily divulged the plan's genesis.

"Well, I saw a discarded large dildo in the prop truck and I asked if I could borrow it, and after not so long of a pause the prop guy happily handed it over to me," the 27-year-old blond beauty added. "I talked to Robert and the producers and made sure that once we got the take and were good, that we could ask for one extra. And unbeknownst to Liam, I just switched out my arm for the dildo."

Needless to say the Aussie actor was a bit red-faced at the fun had at his expense, but Heard said, "He loved it."

Indeed, Hemsworth laughed about it when E! News spoke with him last week.

"It was about that big, a black one," he said, holding his hands nearly two feet apart. "And I pulled it out, and everyone was in on it…except me, yeah."

The experience no doubt helped Liam get into character.

Hemsworth plays a young tech wiz in Paranoia whose prank gets him in trouble with his bosses, who subsequently recruit him for an undercover corporate espionage mission.

The film, costarring Gary Oldman, Harrison Ford, Josh Holloway and Richard Dreyfuss, hits theaters on Aug. 16.