In preparation for the film, Australian director Scott Hicks - whose notable credits include Shine and No Reservations - immersed himself in the stories of returned US servicemen.
"When I started working on the film, I took Zac down to Camp Pendleton - which is a huge military base in California - and we hung out with a group of Marines who are his age, who really had the experience of the character in the story," he says.
"And what was really interesting was how they all connected with the idea of this photograph - they all had some sort of talisman that they carried with them.
"One guy had - what had been a playing card once upon a time - but it had been through the laundromat, and it was just shreds of a card that he'd carried on three tours in his helmet. He lost it one time, and panicked, and thought everything was going to come to an end."
The experience at Camp Pendleton was especially valuable for Efron - whose previous roles haven't required a great deal of emotional depth.
"It really helped Zac," Hicks says. "But it was quite difficult, because you are not of [the soldiers'] world. And they're not sure what your attitude is. So, consequently, they're not quick to communicate with you.
"But I just treated it like I was making a documentary or something, and just asked questions, and kept the conversation going. And gradually, they began to open up. And some of the imagery that they inadvertently provided me with, by telling me their personal experiences, really helped.
"To hear, direct from them, what it's like to be in a Humvee when it hits an IED, and what impact that has on you physically and mentally - it was a very good session that we spent."
And while Efron might seem like an odd choice for a Marine - "he was concerned that his stature wasn't going to be right," Hicks reveals - Nicholas Sparks claims that the actor's appearance and age only serve to further ground the film in reality.
"When most people think of war movies and the Marines, they conjure up Saving Private Ryan - these are people in their thirties," the author says. "It's not like that anymore - we don't have the draft. 95 percent of people who join the military do so right after high school.
"These guys are 18 years old; 19, 20, 21. So when you see ‘em - they're kids. It blows you away.
"I think Zac is 24? He's old!" Sparks laughs. "Actually, he's probably the most realistic portrayal of a soldier that we've seen."
I'm actually glad he brought up the age thing because I think we're really desensitized by the media portrayals of military to how young many servicemen and women actually are and how much they have seen by Zac's age. For example, an Army Ranger named Jared Hagemann killed himself to avoid going back for his ninth deployment. He was only 25. In some ways, I think the most disappointing thing in TLO for me, no matter the quality, will be the fact that, just like in real life, so little attention and care is paid to a returning military member's mental health after some seriously traumatic shit. Jared felt extreme guilt and all he wanted was to be normal again. But Logan, thanks to movie magic, can put his PTSD in a box and forget about it. It's a damn shame, cause they could've said something useful. /soapbox