While Coronation Street star Michael Turner, who plays Kevin Webster, spent the most time in the witness box on Monday, the court also heard from the Mirror's former group political editor, who gave evidence about his time working for the newspaper's former editor Piers Morgan.
Describing an incident that followed a chairman's lunch in 2002, Mr Seymour said he was approached by a colleague as he sat at his desk.
"My colleague started by saying: 'You'll never guess what Piers just said'," Mr Seymour wrote in his statement.
The colleague told Mr Seymour that Mr Morgan had "mocked" the then chief executive of BT, "saying something like: 'You need to tell your customers to change the PIN numbers on their mobile phones from factory settings, because otherwise you can just get into their voicemail messages'.
"My colleague then explained to me how it was done - in fact they had to explain three or four times since I am not technically literate, and I still wasn't sure how phone hacking was done by the end of our conversation.
"My colleague also told me that everyone else at the table heard what Piers had said."
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Mr Seymour said he had "no reason to doubt the veracity" of what his colleague had told him, and said he recalled the conversation "very well" as his colleague "was so shocked".
Asked by MGN's lawyer Mr Munden why he is convinced he was the victim of unlawful information gathering, the actor responded: "There just seems to be a few coincidences, let's say."
He continued: "To be honest, I never really thought about it until someone got in touch with me to point things out."
The actor's claim concerns 28 articles published between 1991 and 2011, covering a range of stories - including a burglary at his home and the births of his children, as well as his arrest for suspected sexual offences, which he was later cleared of, in 2011.
During cross-examination by MGN's lawyer, Mr Turner conceded that some details included in stories he has complained about were available publicly.
He told how his Corrie co-stars thought he was a "mole" - and said in his witness statement that he said at the time he found this "more offensive than being called a sex offender".
In court, he said: "Being a mole or leaking was one of the worst things you could do in our business."
He went on to say he was "mortified" that his former co-star Craig Charles, best known for starring in Red Dwarf, thought he had been leaking information.
Asked to clarify whether he genuinely thought this was worse than being accused of sexual offences, he replies: "It's not, obviously." Mr Turner told the court he said this to emphasise the severity of the accusation and how awful he felt about it.
Mr Turner was later accused himself of a sexual offence, in 2011, but was cleared.
The actor is among more than 100 individuals suing MGN - publisher of the Daily and Sunday Mirror and the Sunday People - for compensation over claims its journalists were linked to phone hacking, so-called "blagging" or gaining information by deception and the use of private investigators for unlawful activities.
The actor's case is one of four representative claims being heard at the High Court in London, alongside similar claims brought by the Duke of Sussex, Hollyoaks and former Coronation Street actress Nikki Sanderson, and Fiona Wightman - the ex-wife of comedian Paul Whitehouse.
MGN denies Mr Turner's claim, arguing there is "no evidence" of voicemail interception or unlawful information gathering relating to him.
Mr Munden has told the court that Mr Turner's case is "particularly weak", with articles in the claim published before phone hacking started, or when it had "significantly dropped off".
Mr Turner's evidence is due to resume on Tuesday and the case is expected to conclude by the end of the month, with a ruling expected at a later date.