Victor Borge never got the memo saying that classical musicians stopped improvising sometime during the middle of the 19th century. So, when a fiddle player wanted to perform a tune that Borge had heard, but never played, Borge simply improvised an accompaniment. It's a bit over the top at points, but then Borge is a comedian. Watch how the two men interact act with one another. There are points where one or the other doesn't quite know what's coming up, so they have to look and listen.
Pay attention at about 3:13, in a slow section (stuck in the middle of all the fast stuff).
It's supposed to return to the up-tempo romp, but the fiddle player strings out a note (which registers on Borge's face) and repeats the slow material, w/ Borge following, of course. At about 3:43 Borge starts a nice counter melody; from 3:54 to the end it's nuts, with a nice counter melody in octaves at about 4:02. Notice the nice hesitation for the very last note, a skillful touch.