Mr Palmer is suing Mr McGowan claiming public comments, including labelling him the "enemy of West Australia", made in July 2020 had damaged the Queensland businessman's reputation.
The legal stoush over defamation claims got underway in the Federal Court on Monday.
The premier has lodged a counter-claim claiming the billionaire defamed him in several interviews.
Mr Palmer is expected to testify this week about the effect Mr McGowan's comments had on him, his barrister Peter Gray SC told the court.
Mr Palmer "has some experience of having harsh things said about him in the public sphere, but these attacks, these accusations, the subject of this case, are in a different league," Mr Gray said.
"They are not just rough-and-tumble name-calling by a politician."
Mr Gray said he expected Mr Palmer would give evidence about the "real and serious impact" of the particular publications on him and those around him.
In their opening on Monday, Mr Palmer's lawyers played footage to the court from media conferences Mr McGowan held in July and August 2020.
Both Mr McGowan and Mr Palmer are expected to testify in person in the Sydney courtroom after Justice Michael Lee earlier said that was his preference especially during defamation proceedings.
"I think that's important for a range of reasons, including my best assessment of matters such as subjective hurt and the like," he added.
In response to concerns by Mr Palmer's lawyer that his client was not vaccinated, Justice Lee said he would not deny anyone access to his courtroom because of their vaccination status.
Justice Lee also refused a request by Mr McGowan and WA Attorney-General John Quigley to give evidence remotely.
Instead he agreed to delay their evidence until later this month to ensure they won't miss parliamentary sittings while self-isolating for seven days when they return to WA.
The trial will run for up to four days this week before recommencing on Saturday, February 26.
Background to the case includes claims relating to the circumstances surrounding WA legislation which prevented Mr Palmer and his company Mineralogy from suing the state for billions of dollars.
Mr Palmer in October lost a separate High Court battle with WA over the legislation preventing him and his company Mineralogy from suing the state.
He was seeking up to $30 billion from WA taxpayers, claiming legislation - introduced by the McGowan Labor government and hastily passed with opposition support - is unconstitutional.
