Steve Arnott told a jury there was something "very fishy" about the footage which allegedly shows Sheridan confessing to visiting a sex club.
Mr Arnott claimed there was too much swearing in the film for it to be the former Scottish Socialist Party leader.
Sheridan and his wife Gail are on trial accused of lying under oath during a defamation case against the News of the World in 2006.
He won 200,000 in damages after the paper printed allegations about his private life.
Mr Arnott – a former regional organiser for the SSP in the Highlands – was giving evidence at the High Court in Glasgow.
The trial again heard about the film that Sheridan's best man George McNeilage sold to the News of the World after the libel trial.
Sheridan asked Mr Arnott – who said he had watched the film – what his opinion of the footage was.
The unemployed 48 year-old replied: "I don't think it is you, Tommy. It obviously sounds a wee bit like you.
"But, obviously a fabrication would not work if it did not sound like you. That is someone doing your public voice."
Mr Arnott claimed the film was "all over the shop" and that Sheridan's tone in reality was "gentler".
He went on: "In the 24 years I have known you, I have only heard you say f*** a number of times.
"This makes you sound like Cartman from South Park."
Mr Arnott said the film "ticked all the boxes" regarding matters "certain people and organisations wanted to prove".
He added: "It is all too neat, all very fishy. Whoever has done it has done a reasonable impression, but it's not you."
Another witness later claimed Sheridan was at a party festival the night he is alleged to have been at a swingers club.
Part-time poet Jim Monaghan told the trial he saw the ex-MSP in Glasgow's Centre for Contemporary Arts on September 27, 2002.
The prosecution alleged Sheridan was at Cupid's in Manchester that day with a number of others.
Mr Monaghan, 47 – once a member of the SSP – recalled approaching Sheridan and that there was "absolutely" no way he was mistaken.
The witness claimed he came forward after the SSP's Allison Kane earlier told the trial Sheridan was not at the festival.
The Sheridans – both 46 – deny the perjury charge.
The trial, before Judge Lord Bracadale, continues.