"The Hunger Games" star was a pre-teen when he first met Kristen on the set of 2005 fantasy film "Zathura: A Space Adventure", in which they played brother and sister, and Josh became fixated on the then-15-year-old actress.
Josh recalled his childhood crush during a chat with fellow co-star Dax Shepard on his "Armchair Expert" podcast, during which he was asked if he was "in love" with Kristen while making the movie.
"A hundred per cent," the 26 year old replied. "She was the coolest person that I'd ever met. She was just so cool... She was playing guitar in her trailer, I was just like, 'Oh my God.' I was like, 'I'm a man, you could date me. I'm 12 years old...'."
"I feel like I played it pretty cool. I don't remember being too like, anxious, but I definitely was like, 'She's beautiful, she's cool as s**t, she's talented.' It was a big time crush at the time."
Josh turned 12 during the Los Angeles-based shoot, and invited the cast and crew to a party at a local bowling alley, where he was sure Kristen was going to make his dreams come true and plant a kiss on his lips as a special birthday gift.
"Kristen kinda walks up to me, and she has her hands behind her back," he remembered. "I'm like, 'Am I gonna get kissed...?' I'm sure I was just freaking out."
However, his hopes were dashed when Kristen instead presented him with a new pet: "She was like, 'So it's your birthday, I wanted to give you a present...'," Josh continued. "She brings her hands out and she's got a turtle - a living, little turtle! A Chilean-something turtle. I'm like, 'Thanks...' She's like, 'They live to be 85, he's less than one (at the moment)!'."
Josh took his new friend, named Bob, back home to Kentucky after filming wrapped, but the actor, who admits he has a "bad record" with small pets, reveals the turtle didn't live to see old age.
"It's been dead for a while," he confessed. "We went out of town in Kentucky and our neighbour was supposed to take care of the turtle. It was winter time, they didn't put the heating pad underneath (his enclosure). Turtle froze to death."