Marcia Gay Harden had no clue Efron was struggling with addiction

Marcia Gay Harden had no idea her Parkland co-star Zac Efron was struggling with addiction issues while they were shooting the John F. Kennedy death drama because he was the consummate professional on set and showed no signs of health issues.

The actress was just as shocked as everyone else when news leaked about the young star's alleged abuse troubles, which prompted him to seek treatment in rehab earlier this year (13).

And she admits the fact that he made sure his problems didn't impact on the film or his castmates has made her respect Efron even more.

She tells news show Access Hollywood Live, "He was definitely together. Like all the boys (in the film)... they had a couple of drinks at night, they had a good time and I was the mother hen because I don't drink... but we got along super well. He prepared, he was open to hearing the process.

"He's the nicest person and he's working to be a serious actor."

Harden admits she admires her co-star for putting his health before his career: "Anybody who changes their life by addressing their issues, I have down and out respect for them.

"He didn't use his rehab to boost his ratings; he took care of it privately and elegantly and I love the guy."