My father once told me of one of the violin teachers he studied with as a young man. He taught at a certain level, violinists aspiring to a professional career, and prospective pupils would have to pass this audition: to play a scale of C major.
That might sound an easy thing to do, but the candidate would have to start from the open G string, rise three octaves through the full range of the instrument, and back down again, a semibreve to each note. No vibrato. With a metronome mark of ♩= 60, a semibreve is four seconds long, and the whole exercise takes about three minutes.
Any fiddler can busk a Paganini caprice, but to hit every note exactly, with no vibrato to hide behind, and maintain an even tone over the whole length of the bow and the whole register of the violin, under exacting scrutiny: that is a test of character as much as musicianship.
Image credit: caseantiques.com.