The 36-year-old actor got ripped to play the professional wrestler Kevin Von Erich in the new movie and felt his look — which included a wig and skimpy wrestling shorts — was "ridiculous" initially until things came together.
Speaking to The Hollywood Reporter at the film's Los Angeles premiere on Monday (December 11), Zac said: "In the best way possible, I did not look anything like me. It was jarring at first, but we kind of put it together piece by piece.
"Individually, the wig by itself looks kind of ridiculous; by themselves, the clothes look pretty crazy. When you put them all together in the sets that our team built and get out there in the ring under those lights, it looks like the Von Erichs, man, and that was our goal. I think it added a lot to it."
The film tells the story of the Von Erich brothers, who made wrestling history during the 1980s but have been cursed by tragedy, and Zac explained how it felt "incredibly gratifying" to see the real-life Kevin's reaction to the picture.
The High School Musical star said: "Over the course of filming this movie, that was pretty much all I could think about. It's something that you try and kind of push back a little bit, you're trying to just do the work and get it done, but it's always in the back of your mind, 'What's Kevin going to think of this?'
"To meet him, to spend time with him, to have him be out here with us on the carpet, it feels incredibly gratifying, more so than anything I've ever done in my entire life. I'm so proud, and I'm just honoured that he's all a part of this. It's all because of him."