Speaking to audiences, spontaneous applause (including in the middle of pieces), and on-the-spot improvisation were commonplace, with performers wooing audiences with their technical and interpretational prowess. That was real classical music.
But all of that changed in the 20th century when "rules of concert etiquette" began incorporating themselves into performances. Suddenly, clapping mid-piece was unacceptable. Applause between movements, or parts, of pieces became that way as well. Regular concertgoers would shoot glares of disdain toward anyone violating such rules.