The 25-year-old Hollywood actor, who secretly completed a stint in rehab about five months ago, was praised by Marcia for his recovery efforts.
"In my opinion, Zac Efron is a total hero," said Marcia, speaking to E! Online. "Him seeking help encourages other people with addictive issues to seek help. It's brave of him.
"I think only a coward lives in denial, and he is not a coward. He's a hardworking go-getter and I love him."
Zac clearly feels the same respect for his co-star, and posted a photo of the duo on the set of Parkland.
"Marcia Gay Harden @Beloving2 is a SAINT... So lucky to be working with her. She makes me want to be a better man. – Z," wrote the actor alongside the image.
Zac, who reportedly sought treatment for supposed cocaine and alcohol addictions, kept quiet about his stay in rehab. The news that he had checked himself in shocked fans when it emerged earlier this month.
The former High School Musical star has been seen out and about promoting his latest film, and is said to be "happy, healthy and not drinking".
In his latest film, Zac plays Jim Carrico, the real-life doctor who battled to save John F. Kennedy at Parkland Memorial Hospital after the president was shot in 1963.
"It's a very surreal experience for me," Zac said, about working on the film. "It feels very mature and really interesting. It's a whole different kind of filmmaking and I feel blessed to have been a part of it."
Parkland will be released in UK cinemas in November.