So well she acted all and every part
    By turns -- with that vivacious versatility,
Which many people take for want of heart.
    They err -- 'tis merely what is called mobility,
A thing of temperament and not of art,
    Though seeming so, from its supposed facility;
And false -- though true; for surely they're sincerest
Who are strongly acted on by what is nearest.

This makes your actors, artists, and romancers,
    Heroes sometimes, though seldom -- sages never;
But speakers, bards, diplomatists, and dancers,
    Little that's great, but much of what is clever;
Most orators, but very few financiers,
    Though all Exchequer chancellors endeavour,
Of late years, to dispense with Cocker's rigours,
And grow quite figurative with their figures.

                        - Lord Byron, Don Juan

From his notes:
In French, "mobilitÈ. I am not sure that mobility is English; but it is expressive of a quality which rather belongs to other climates, though it is sometimes seen to a great extent in our own. It may be defined as an excessive susceptibility of immediate impressions -- at the same time without losing the past; and is, though sometimes apparantly useful to the possessor, a most painful and unhappy attribute.