"If you believe that . . . you'd believe anything," he growled, those eyes open just enough to fix the interviewer in a glare of smouldering intensity.
This was supreme late-night television in the sort of slot often reserved for examination of the psyche of serial killers and other dangerous obsessives, but this was a sports profile.
The words elic iting the response had come originally from one Alex Ferguson, arguably the greatest football manager of all time, while the skill of cameraman and editor provided a perfect juxtaposition between Roy Keane's verbal response and what we could see on our screens.
"His eyes started to narrow, almost to wee black beads. It was frightening to watch," is the passage from Ferguson's book, describing the most successful of his captains, that had just been read to Keane.
Keane & Vieira - Best of Enemies had been billed as an examination of a great sporting rivalry. To some extent it did that, but its most fascinating aspect was the depth of insight into what drove the Irishman, when measured against the former Arsenal man, a fearsome competitor in his own right, who came across as phlegmatically well-adjusted.
No opportunity to goad Vieira was missed, but neither was any opportunity to get any sort of dig in at a man who might once have been seen as Keane's most kindred spirit, his former boss Ferguson. In among it all, though, one passage should be shown to everyone involved in professional sport, in particular those who earn their money in those sports where readiness to commit blood and sweat to the cause are far more important than talent and, for that matter, tears.
It concerned the Champions League semi-final of 1999 and the moment Keane knew he would not be taking part in the final when a booking for a late tackle meant he would be suspended if United got through.
"In the very same stadium where Gazza shed semi-final tears, Keane oozed defiance as United rolled on to victory," the narrator told us, pointedly. "One night in Turin headlined for better and for worse, by one man."
It then cut to the aptly brutalist, abandoned warehouse setting chosen for the conversations with and between the two protagonists. "Pounding every blade of grass, competing as if he would rather die of exhaustion than lose, he inspired all around him. I felt as if it were an honour to be associated with this player," we hear from Gabriel Clarke, the off-camera interviewer.
His intonation made it clear that this was being read from a text and Keane's suspicion about where this was going was visible even before Clarke added: "Sir Alex Ferguson on Roy Keane versus Juventus, 1999." There was a brief attempt to be seen to be taking that in good spirit as Keane noted rhetorically, with a trace of a smile: "He didn't put that in his last book did he?"
Confirmation that it had, in fact, been written in Ferguson's previous autobiography, written before these two most intense of individuals parted company professionally and emotionally, led to those eyes darkening once more as he sought another way of getting back at his old manager. In doing so, though, Keane inadvertently offered a message to all involved in professional sport.
"Stuff like that almost insults me. What am I supposed to do? Give up? Not cover every blade of grass? Not do my best for my team-mates? Not do my best for my club?" the Irishman asked.
Warming to his theme the jaw clenched ever more firmly and those eyes narrowed once more. "I actually get offended when people throw quotes like that at me as if I'm supposed to be honoured by it. It's like praising the postman for delivering your letters. He's supposed to isn't he? That's his job. My job is to try to win football matches for Man United," he said.
Keane is an extraordinary character. The fiery intellect that makes him such a magnificent competitor also, at times, results in desperately poor judgment.
He admitted in the course of the programme to regretting having walked out on Ireland's World Cup campaign. Perhaps even more telling, though, was the ridiculous selection of a Manchester United "Dream Team" that did not contain Paul Scholes, Ryan Giggs or Wayne Rooney but did include Paul Ince and David Beckham.
Set aside any other consideration, though, and this footballer offered a message that sometimes seems, in particular, to have been forgotten in what likes to think of itself as the tougher world of Scottish rugby where similarly intense characters such as David Sole, David Leslie and Jim Telfer used to provide the drive.
Let us, then, never again hear from Scottish international rugby players - we did yet again last month - claims that they deserve credit for the amount of effort they put in.
As Roy Keane so rightly said, that is the minimum requirement within the job description. Talent and judgment must be assessed more subjectively, but failure to work hard represents nothing less than a gross misdemeanour for those in that line of employment.