The Tommy Sheridan perjury trial has heard that the ex-MSP does not feature in a secret video in which he allegedly confessed to visiting a sex club.
Ian Fitzpatrick said he "knew for a fact" Mr Sheridan was not involved while Philip Stott later claimed it was a "poor caricature" of Mr Sheridan.
The politician and his wife Gail, both 46, deny perjury.
They are accused of lying during his successful defamation case against the News of the World in 2006.
The former Scottish Socialist Party (SSP) leader won £200,000 after the newspaper printed allegations that he had committed adultery and visited a swingers' club.
After the court action, Mr and Mrs Sheridan were charged with perjury.
The High Court in Glasgow again saw part of the video that Mr Sheridan's best man George McNeilage sold to the News of the World for £200,000 after the defamation case.
The footage is said to feature Mr Sheridan admitting to Mr McNeilage allegations about his private life.
The former politician has repeatedly claimed he was not involved and that the film is concocted.
Trade unionist Mr Fitzpatrick told the court how he joined the SSP in 2002 and later got to know Mr Sheridan.
He said he watched the video bought by the newspaper in October 2006.
Mr Sheridan, who is representing himself, said four Crown witnesses had stated that it featured him. He asked the witness what he thought.
Mr Fitzpatrick said: "Absolutely not. There are just so many things. Knowing you for the length of time that I have, I know for a fact it is not you."
The 36-year-old father denied that he would lie for Mr Sheridan and risk going to jail.
Prosecutor Alex Prentice QC later asked: "You said that it is not him in so many ways. Tell me one."
Mr Fitzpatrick replied: "The tone. The thing that struck me when I first watched it in terms of behaviour was the long pauses and the swearing.
"It was in stark contrast. It is not something that I am aware of in all the times I have been in his company.
"He is somebody who always knows what to say and always says it in a very eloquent way."
Mr Prentice then asked: "So, if it is not him then it must be an actor?"
The witness replied: "I have no idea. I would say so, yes."
Mr Fitzpatrick also recalled standing near to Mr Sheridan when he was interviewed on TV, along with Scottish News of the World editor Bob Bird, after the film was released.
The jury was also shown a clip of that interview.
In it, Mr Sheridan claims: "I certainly was at no meeting in relation to the creation of this audio tape.
"At the end of the day, what they have done is concoct a spliced version of a tape which may include my voice, but certainly includes somebody else's voice."
Mr Fitzpatrick told the court how he had refused to give a statement to police after consulting with the office of Mr Sheridan's lawyer, Aamer Anwar.
Philip Stott, a founding member of the SSP, was also quizzed about the video and said he was "absolutely certain" Mr Sheridan's voice was not heard.
Mr Prentice asked the 46-year-old: "You would agree that it sounds like him?"
Mr Stott replied: "No, not at all. I would probably best describe it as a poor caricature. It is definitely not Tommy Sheridan. It definitely does not ring true."
The witness, who has known Mr Sheridan for 26 years, said there were two reasons why he did not give a statement to police when contacted before the trial.
He told how he too consulted with Mr Anwar's firm, adding that he also had concerns about the conduct of Lothian and Borders Police.
Mr Stott said he believed there was a "vendetta" against the Sheridans.
He later said: "I thought the measures taken by Lothian and Borders was unprecedented.
"The money, the way they have conducted themselves. They have not been impartial the way they have approached this investigation."
Mr Sheridan later told the jury he hopes to end his defence case on Thursday or Friday of this week.
Earlier, the court heard from Charlotte Ahmed, a 50-year-old chemistry teacher from Glasgow.
She had been an SSP member at the time when Mr Sheridan was alleged to have admitted visiting a swingers club during an executive meeting of the party on 9 November 2004.
A total of 16 members of the executive previously told the court that Mr Sheridan made the admission at the meeting.
Ms Ahmed, who was not present at the gathering, said she had not seen the minutes from the meeting at a later meeting on 24 November.
Mr Sheridan denies making false statements as a witness in the defamation action on 21 July 2006.
His wife denies committing the same crime on 31 July 2006 after being sworn in as a witness at the civil jury trial.
The trial, before Lord Bracadale, continues.