The 22 year old gets on well with younger sibling Dylan and thought about their friendship often while he was on set of the emotional drama.
And Efron used the opportunity to be affectionate with his onscreen brother, played by Charlie Tahan, as a substitute to professing his feelings for Dylan.
He tells BBC Radio 1, "We're not as lovey dovey as the brothers in the movie. We're pretty competitive and we're dudes.
"I think that's the reason it was fun to have a fake little brother because I could really go up to him and say I love you and all that stuff because me and my brother don't do that - it's unsaid.
"We're dudes so you kinda can't! I was able to let all of that come out - I was thinking of him (Dylan) a lot during filming."