This incident does not actually occur in the novel. It purportedly happened to Curley's wife before she was married. She tells Lennie about it in the barn shortly before he accidentally kills her. She does not specify what kind of a "show" was traveling through Salinas or what kind of "actor" the man was. There is a good possibility that he was just some stage hand or other roustabout. The "show" might have been nothing more than a carnival. Curley's wife tells Lennie:
But my ol' lady wouldn' let me. She says because I was on'y fifteen.
Lennie probably doesn't understand half of what she is telling him or why she is doing so. She is lonely and unhappily married. She is looking for someone to confide in. She thinks the "actor's" offer to take her away would have been a great opportunity, but the reader understands that the man was only trying to take advantage of a hopelessly naive young girl.
Steinbeck does not specify Curley's wife's exact age, but she may be as young as sixteen. Steinbeck may have made her very young and not fully developed in order to make it more credible that Lennie could kill her so easily by shaking her as he did. Her extreme youth would also explain why she didn't know better than to get too close to someone strong and unpredictable like Lennie.