Tour guides are the pastorate of the art historical profession. An art historian unable to lead a compelling tour is missing something essential, as is a theologian incapable of preaching a compelling sermon. Likewise, guiding tours (and pastoring churches) is a generally undervalued pursuit - albeit one arguably more important than formal art history (or professional theology). But undervalued or not, a tour, and sermon, is where the rubber of one's academic knowledge hits the road of public concern.
Being a good tour guide is like being a good waiter. The best waiters know how to strategically recede, foregrounding the customer's encounter with food and their chosen company - but at the same time, good waiters know when to assert themselves to enhance the dining experience. In the same way, the best tour guides assert themselves when necessary to stimulate an inattentive or perplexed audience, but the best tour guides never assert themselves too much.
The skill of delicately leeching off an ongoing tour at a given monument or museum can seriously enhance one's trip. On my recent sojourn through fair Italia, the Temple University professor I heard from at the Borghese Gallery in Rome ("the queen of private collections") was deeply informative, and I seemed to be the only one among his class that was listening. Likewise a University College London Professor's tour of Tinotoretto's Scuola San Rocco was one of the finest art history lectures I have witnessed. But, one of course does not need be a professional art historian to lead a tour. What is necessary is a jealously passionate love for one's subject and an ability to impart that enthusiasm, grounded by an accurate mastery of the historical details. The best tour guide I have ever had (at the Wurzburg residence) was probably not a professional art historian. The second best, at the Scavi tour under St. Peter's, was. The degree is secondary. Both these guides took risks to engage their audiences, and both succeeded.
At a recent Gothic conference at Princeton, Columbia's Stephen Murray (check his websites) departed from the standard academic routine, and attempted to construct a theory about how monuments need interpreters, else those monuments remain mute. He added that this new model could include spiritual meaning, which should not just be the domain of Abbot Suger. It was a refreshing break from what I'll call the "Panofsky Piñata" paradigm in which academic conference presenters take turns swinging away at an earlier generation of art historians, complete with blindfolds but without the candy. Murray's was a wonderfully complex presentation that sought to fit the new wine of active monument interpretation (i.e. tour guides) into the old wineskin of traditional art history. It might have been enough to cause some conference participants to put down their sticks.