Lead-in to a play "The prologue, Greek prologos (meaning: before word), is an opening of a story that establishes the setting and gives background details.
Generally speaking, the main function of a prologue tells some earlier story and connects it to the main story. Similarly, it is serves as a means to introduce characters of a story and throws light on their roles. In its modern sense, a prologue acts as a separate entity and is not considered part of the current story that a writer ventures to tell." https://literarydevices.net/prologue/
Warm up act
:"the act or an instance of warming up; also : a preparatory activity or procedure" https://www.merriam-webster.com/dictionary/warm-up