What do we mean when we say an artist like Jackie Evancho is "expressive" ?

Actually, "Expressive" is the wrong word for what I'm trying to talk about, because it implies that the performer/artist is expressing something inside of them, and I'm not talking about the performer's inner life--only about their ability to make the viewer/listener/reader feel thoughts/emotions strongly.

Thus while a convicted criminal may weep while apologizing to his victims or their families during a trial's sentencing phase, you can't know whether they're tears of remorse or of self-pity.

And I think I've heard that some porn stars think about playing with their pet dog, or eating candy, while they're "acting."

Ultimately, until brain science gets more advanced, I can't tell what's going on inside your head and vice-versa, whether your face and microgestures are considered expressive or impassive.

Putting that aside, though, we can distinguish between performers who make us feel admiration for their skill as performers vs. performers who make us feel, period.

Like the probably apocryphal saying that when the ancient Roman orator Cicero spoke, people would say "How well he speaks," but when Greek orator Demosthenes spoke, people would say "Let us march on Sparta."

That, for me, is the difference between Pavarotti and Domingo. It could be that Pavarotti was feeling powerfully moved when he sang, while Domingo thought of eating candy. But many listeners have felt as I have about what we experience and feel when listening to these two operatic tenors.

And there's a whole school of thought that advocates how art should not make you feel deeply, since that can often be distressing, after all (think of the grim choral ending of Threepenny opera, which points out that the hero's near-magical rescue from the gallows doesn't happen in real life). Instead it argues for aesthetic distance--art through binoculars--in which you're amused, diverted, entertained, but not gut-wrenched or even overjoyed. Just pleased. That's exactly what the ancient Greek Epicurean school of philosophy taught, saying you should avoid life's peaks because you spend most of your life in the valleys and those peak experiences ruin the valley time.

"How well she sings" vs. "I wept."

Though that varies by viewer. Jackie has the latter effect on my, but the former effect on my brother, for example. Her singing makes him feel close to nothing, even though he recognizes her skill.

Poor guy.

Can you quantify this? I can imagine coming up with some kind of quantification of what looks like emotional expressiveness, based on microgestural indices from taking readings off the 300-odd facial muscles humans have.

Or you can go at it sociologically. Say, interview 100 opera lovers after having them view opera clips of Pavarotti and Domingo singing several of the same arias, and see whether you get statistically significant results, using standard statistical methodologies.

Verisimo and bel canto schools of opera focus on emotional engagement and beautiful singing for its own sake respectively. Same thing is true for pop music, with the fun stuff vs. the heartfelt stuff. Like Christina Aguilerea's Genie in a Bottle vs. You're Beautiful (I think that's its title).