Some conductors believe, above all, in The Rehearsal. They balance and tune chords, bring out some voices and subdue others, they charm and educate players with spoken poetry and imagery to achieve various effects. They even make adjustments in response to the acoustic of a particular hall.
Others do plenty of preparation in rehearsal, but the main thing they bring to the party is a performance pumped with energy.
Conductors operating on the highest level are a substantive amalgamation of the two - they did their homework before curtain time, and they have the skillful gestures to write new ideas in performance and the sensitivity to react spontaneously to events (good and bad) unfolding in ensemble.
Recent Philadelphia maestros who fall into this highest order are Wolfgang Sawallisch, Simon Rattle and Vladimir Jurowski.
Yannick Nézet-Séguin may be poised to join their ranks, but so far, he has proven mostly to be an extremely charismatic manifestation of adrenaline.
Deeply considered interpretation isn't a priority at the moment. He's too busy seizing glamorous debuts. And who could blame him? The Metropolitan Opera, Vienna Philharmonic and Berlin Philharmonic beckon.
But now that classical music's jeune prodige has a music-director post with a major orchestra - an ensemble finer than any he's headed full-time so far - his ability to deepen interpretative statements hinges on resisting career temptations. It's time for him to do more with less, and to make Philadelphia his main priority. Can he? Will he?
No one can predict the future development of conductors. I can't come to any conclusions about Nézet-Séguin yet, even after a handful of encounters. That the Philadelphia Orchestra can, and did, on the evidence of just two programs, shows breathtaking daring.
The Brahms Violin Concerto I heard him lead in February at Lincoln Center with his own orchestra, the Rotterdam Philharmonic (a concert also heard by the search committee), was only partly imprinted with a discernible stamp from the podium. His Franck D Minor Symphony with the Philadelphia Orchestra was fine as far as it went, though hardly brimming with revelation.
There's a singing quality to some of his work that is most promising. It came through somewhat in the Tchaikovsky Symphony No. 6 at his Dec. 2008 Philadelphia Orchestra debut.
Fast tempos are a running theme. A Mendelssohn Symphony No. 4 ("Italian") with the Mostly Mozart orchestra in 2009 went by too quickly to develop dynamics and phrase-shaping - a missed opportunity in melody-rich Mendelssohn.
Nézet-Séguin, 35, is very much a conductor for our age - dancerly, a telegrapher of emotion by way of facial expressions, and the master of elegant gestures that bring audiences along visually.
Look at me, listen with your eyes, this young conductor urges. But in the end, it's the listening the ears do that counts most. Can he operate on both levels? Can he relate colloquially to the masses while simultaneously pleasing hardcore classical fans?
We'll be listening hopefully, hopefully for great things.