Armstrong will break his silence over damning accusations of his drug doping on the Oprah show next Thursday but Millar, a member of the athletes’ commission for the World Anti-Doping Agency, said the American needs to come clean in a proper arena.
Millar said: "Only Lance would get to have his moment of truth, if that's what it will be, in front of Oprah Winfrey.
"It is not sitting in front of a judge or a disciplinary hearing being properly questioned about the things he has done wrong. I doubt very much it will be a proper interrogation.
"My biggest concern is that it will be completely stage-managed, that he will just be 'given the ball', and that it will all be about his emotions rather that concentrating on exactly what he did wrong.
"The question should also be asked whether he is getting paid for going on the show."
The show's producers have yet to respond to an email asking whether Armstrong will be paid for his appearance.
Millar himself served a two-year ban after admitting doping in 2004 since when he has become a campaigner against drugs in sport.
Armstrong was stripped of his seven Tour de France titles by the International Cycling Union (UCI), following a report by the US Anti-Doping Agency which concluded the 41-year-old and his US Postal Service team had run "the most sophisticated, professionalised and successful doping programme that sport has ever seen".
The Oprah show will be Armstrong's first formal interview since he was banned for life by the UCI with the Oprah Winfrey Network claiming in a statement that "Armstrong will address the alleged doping scandal, years of accusations of cheating and charges of lying about the use of performance-enhancing drugs throughout his storied cycling career."
The interview will take place at 9pm ET on Thursday, January 17 (0200amGMT on Friday, January 18) and is scheduled to last for 90 minutes.
It will come only several hours after the full details are announced of the 2014 Tour de France's 'Grand Depart' in Yorkshire. The New York Times claimed at the weekend that Armstrong was close to admitting to the damning report from USADA.
The Texan, who did not co-operate with the USADA investigation, has remained silent since the sanction, although he opted not to appeal the decision.
Armstrong, who has shown an interest in competing in triathlons, also removed mention of his seven Tour wins on his Twitter profile.
Asked whether the 41-year-old was set to come clean about his drug-taking past Armstrong's lawyer, Tim Herman, told the New York Times: "Lance has to speak for himself on that."