All 4 Uses of

impromptu
in
The Magic Mountain

Both Behrens and Joachim looked at him to see if he was not ashamed of himself for these impromptu babblings.†
Chpt 5.6 *

In the most ticklish situation he had managed— with good grace and quite impromptu—a vindication of drink; he had, moreover, just in passing, brought the conversation around to "civilization," of which, to be sure, little was evident in Mynheer Peeperkorn's primitive, menacing pose; and finally, by asking his question, had relaxed that grandiose pose—it would have been quite inappropriate to respond with a raised, clenched fist.†
Chpt 7.3

It was a mere digression, an instructive example to sensitize them to the basics of life, an impromptu fantasy, which he then dropped to turn his official and emphatic emotional engagement back to the immediate demands of the late hour's festive abandon.†
Chpt 7.3

and Peeperkorn would accompany these impromptu meals with grand tattered phrases, or he might demand a tale from Anton Karlovitch Ferge, the good-natured martyr to whom all higher things were utterly foreign, but who knew some very down-to-earth details about the manufacture of Russian galoshes—sulfur and other ingredients were added to raw rubber, and the finished boots were then lacquered and "vulcanized" at temperatures over two hundred and fifty degrees.†
Chpt 7.4

Definition:

without advance preparation