Johnny was accused of domestic violence against Amber in an op-ed that she wrote for the Washington Post, and he is suing her for $50 million (£38 million) for defamation.
In a new interview with Britain's The Times newspaper, Oscar-winning actor Mark - who worked with Depp on 2019 movie "Waiting for the Barbarians" - insists his colleague and pal is "a very loving and very sensitive and very trustworthy person."
"(He is) intensely, intensely shy. You know? Like Daniel Day-Lewis and so many great actors are," he explains. "The accusations that were around at the time we were making the film were of physical violence. And he obviously denied this and I found his denial completely believable. It didn't seem to be part of his character at all."
The "Pirates of the Caribbean" star sued his ex-wife after she referred to herself as "a public figure representing domestic abuse" in a Washington Post article in 2018.
He claimed he had been defamed in the piece even though Heard didn't name him, stating it suggested the actress was a domestic abuse victim and he "perpetrated domestic violence against her."
The couple split in 2016 after Heard accused Depp of being abusive throughout their 15-month marriage.
Depp has always denied her claims that he was violent towards her and insists he has proof that all of her allegations against him are bogus. Heard responded by offering up details of several alleged instances of abuse at the hands of her ex.