In an interview with The Hollywood Reporter, Zac, 26, said the scuffle happened when he tried to defend his friend after getting stranded in a rough part of LA.
"I had a friend come pick me up late at night — we were looking for a place downtown to get a bite and catch up," Zac revealed. "We were having trouble finding somewhere — a lot of places were closed — and the car ran out of gas off the 110. It was ridiculous. We had to pull over...
He continued, "A homeless guy, or vagrant, tapped on the driver's side window. Before I knew it, (the friend) was out of the car and they started fighting. I saw that (the homeless man) was carrying some sort of knife, or shank, and I got out of the car to disarm him."
Zac added, "At some point, he dropped the knife and I got hit pretty hard in the face— and almost instantly the police were there to break up the fight."
During the interview, Zac also opened up about his time in rehab.
"I was drinking a lot, way too much," he admitted. "It's never one specific thing. I mean, you're in your 20s, single, going through life in Hollywood, you know? Everything is thrown at you. I wouldn't take anything back; I needed to learn everything I did. But it was an interesting journey to say the least."
Zac completed his secret stint in rehab back in April 2013 for alcohol and cocaine addiction, and has since joined Alcoholics Anonymous.
"I just started going," he said. "And I think it's changed my life. I'm much more comfortable in my own skin. Things are so much easier now."