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.
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."
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.
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."
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."
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.
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
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).