Amber Heard says her jealous ex-husband kicked her during a flight because he believed she was having an affair with her co-star James Franco, whom he “hated”.

Heard took to the stand for her second day in the US$50 million ($69 million) defamation trial brought by Pirates of the Caribbean star Johnny Depp.

Heard testified that in 2014 on a flight from Boston to Los Angeles, Depp was angry about romantic scenes she shared with Franco and repeatedly questioned her.

Franco was her co-star in Pineapple Express and The Adderall Diaries.

Heard told the court Depp, who she said was badly affected by jealousy and drugs, kicked her in front of other people on the plane.

“I feel this boot in my back. He just kicked me in the back. I fell to the floor and caught myself. No one said anything no one did anything,” Heard testified.

The court also heard a recording allegedly of Mr Depp “howling” on the flight, which Ms Heard said showed him affected by drugs.

She said Depp later apologised for the incident and took steps to undergo detox from drugs.

Amber Heard during her second day of emotional testimony. Photo: AAP

Jurors have repeatedly heard about the cross-country flight, but in Depp’s version of the trip he claims he took Oxycodone pills and fell asleep to avoid Heard.

In Thursday’s testimony, Heard outlined what she alleged was Depp’s physical abuse in their relationship during which he was affected by drugs and alcohol, leading to her lashing back.

“Nothing I did made him stop hitting me. Nothing. I tried for over a year, maybe two, of just not responding physically, not responding verbally, just staring at him, I tried to freeze, I tried to go to a different place,” she told the court in Fairfax, Virginia.

“I would try then lashing out verbally. I would try to threaten that if he hit me again that I would call the police.”

In his own testimony earlier in the trial, Depp claimed he had never struck Ms Heard, or any woman, but described Heard’s “need for conflict”.

Amber Heard appeared deeply distressed while recounting incidents to the court. Photo: AAP

Heard’s second day on the stand followed her opening testimony on Wednesday in which she painted a picture of a “magical” relationship before it turned violent.

Depp’s first physical outburst, Heard said, came after she asked what was written on one of his faded tattoos. She said he replied “Wino,” and she laughed, thinking it was a joke.

“He slapped me across the face,” Heard said.

“I didn’t know what was going on. I just stared at him.”

Heard said he slapped her twice more and said “You think it’s funny, bitch?”

Tattoo fight ‘didn’t happen’

Depp, 58, testified earlier in the trial that the tattoo fight “didn’t happen” and that he never hit Heard. He argued that she was the one who was the abuser.

The star of the Pirates Of The Caribbean films and others is suing Heard, 36, for $US50 million ($69 million), saying she defamed him when she claimed she was a victim of domestic abuse.

Heard has countersued for $US100 million ($138 million), arguing Depp smeared her by calling her a liar.

Under questioning by her lawyers, Heard said Depp assaulted her “several times,” usually when he was drinking or using drugs.

During a weekend away with friends in May 2013, Heard said, Depp accused her of inviting what he perceived as suggestive advances from a woman.

That evening, Heard said Depp ripped off her underwear and stuck his fingers “inside” her in a “cavity search” for drugs.

Johnny Depp has a different version of events. Photo: AAP

Hunt for cocaine

Earlier, psychologist Dawn Hughes testified that Heard had told her Depp had put his fingers up her vagina in a hunt for cocaine.

The case hinges on a December 2018 opinion piece Heard authored in the Washington Post. The article never mentioned Depp by name, but his lawyer told jurors it was clear Heard was referencing him. The couple’s divorce was finalised in 2017 after less than two years of marriage.

Depp, once among Hollywood’s biggest stars, said Heard’s allegations cost him “everything”. A new Pirates movie was put on hold, and Depp was replaced in the Fantastic Beasts film franchise, a Harry Potter spin-off.

Opinion protected under free speech

Heard’s lawyers have argued that she told the truth and that her opinion was protected free speech under the US Constitution’s First Amendment.

A state court judge in Virginia’s Fairfax County, outside the nation’s capital, is overseeing the trial, which is expected to last until late May.

Less than two years ago, Depp lost a libel case against the Sun, a British tabloid that labelled him a “wife beater”. A London High Court judge ruled he had repeatedly assaulted Heard.

Depp’s lawyers have said they filed the US case in Fairfax County because the Washington Post is printed there. The newspaper is not a defendant.

-Reuters