The guest's initial reaction to the beginning of the Mariner's story in Samuel Taylor Coleridge's poem "The Rime of the Ancient Mariner" is one of curiosity and interest. The Mariner approaches the wedding guest and stops him, telling him that he must listen to his story. At first, the guest is annoyed and tries to walk away, but the Mariner's "glittering eye" and "skinny hand" hold him in place.
As the Mariner begins his tale of a disastrous journey at sea, the guest becomes increasingly captivated by the story. He is drawn into the Mariner's narrative as he describes the stormy weather, the appearance of the albatross, and the subsequent events that lead to the Mariner's crewmates dying one by one. The guest becomes so engrossed in the story that he forgets about the wedding he was on his way to attend.
Overall, the guest's initial reaction to the Mariner's story is one of skepticism and annoyance, but he quickly becomes fascinated by the tale and is unable to resist listening to it until the very end.