What is the style of singing without prior preparation or practice in the leader-chorus form.?

The style of singing without prior preparation or practice in the leader-chorus form is often referred to as "improvisational" or "spontaneous" singing. In this format, a leader sings a phrase, which is then echoed or responded to by the chorus. This style emphasizes creativity and interaction, allowing for immediate expression and participation, often found in folk, spiritual, or call-and-response traditions. The lack of preparation adds an element of freshness and unpredictability to the performance.