Keller, who joined Millwall from an American college, is completing his sociology degree, and on the question of sports ethics he chose a potentially fascinating subject: infidelity and the professional athlete. Which explains his interest in the extra-curricular activity of the fit and famous.
As a result, the conversation, in advance of tonight's FA Cup replay with Chelsea, took a welcome turn away from whether zonal marking or a sweeper defence would best succeed. "The question I was trying to answer was whether, by nature, professional athletes cheat more on their wives or girlfriends, or whether it is just the case that the opportunity is there for them. Also there is the publicity factor, because you are less likely to read about it, if it involves somebody less well known.
"There wasn't the time to undertake any personal research," he said, without a trace of a smile. In any case he is happily married. "So it was basically a matter of collating information from American sports. There was the Magic Johnson situation, and Wilt Chamberlain, the former basketball star who is reckoned to have slept with 10,000 women.
"You know, in America it would not be unusual for Magic Johnson to travel to an away game and find 25 women waiting for him outside the hotel. I talked the subject over with some of my team-mates at Millwall, not to pry into their own personal experiences but to find out what would happen if they went to a night-club with a friend, who was not a footballer, who would have the most opportunities?
"Of course the player does, but it also true that the friend may have more opportunities through being with the player than would otherwise be the case. The fact that they were with someone who other people recognised seemed to be a help."
At which point the thought occurred that, having won at Nottingham Forest in the Coca-Cola Cup earlier in the season, and at Arsenal in the third round of the FA Cup, Millwall rather enjoy playing away from home but it was a poor-taste joke, best resisted. It makes the point, though, that Keller and his team have no reason to fear the Stamford Bridge replay.
"The first game with Chelsea was similar to the first with Arsenal - pretty tight with few chances. After Highbury we certainly don't think we've lost any advantage by going to a replay."
The 25-year-old, in his third full season in English football, has proved to be just about the most consistent of the foreign legion who challenge the assumption that no country produces goalkeepers like England. There has been no looking back since he first impressed Millwall in a trial even though it cost him the chance to represent his country, the host nation, in last summer's World Cup.
Bizarrely, the United States denied themselves his big-game experience because, he suspects, his preferred choice to learn his trade at the sharp end, instead of committing himself to a two-year World Cup preparation at home, upset the powers that mattered.
Promises were made and not kept, but while many feel he was harshly treated, Keller refuses to say so publicly. "I just found it slightly strange that there was two of us - me and Jurgen Sommer of Luton - holding down regular places in one of the hardestleagues of all and supposedly competing for only the third goalkeeping place."
It was a contact of Bruce Rioch, Mick McCarthy's predecessor at Millwall, who first alerted the club to Keller's potential. You wonder how much he knew of the club's reputation as a home to some of the most infamous supporters around.
"I had heard of Millwall because I had a friend who used to send me tapes of English games, but it came as a bit of a shock when I telephoned one of my professors, a Scotsman, in America telling him what I was going to do. `Millwall!' he said. `They're crazy down there.' When I came over and saw the old Den for the first time I remember thinking... this is kind of interesting. From what I was used to, it definitely was not the Rose Bowl in Pasadena."
South London soon felt like home, however, and having his girlfriend Kristin, now Mrs Keller, around helped him to settle down. Millwall's transfer to a new ground confirmed that he had joined a club on the move. From time to time, his outstanding displays have brought him a mention in connection with some of the biggest clubs and, though it would take a lot of money now for Millwall to let him go, Keller believes at some time in the future the lure of experiencing football elsewhere in Europe or Japan will be irresistible. Then it will be back home to the States and a new life.
"There's a big market for the game over there now and hopefully the right situation might come along for me in, say, about 10 years. It could be as a coach, a player-manager, an owner, a secretary, who knows. Whichever way someone wants to make use of the education and experience I have gained over here."