Initially, Leah hadn't planned to have anything to do with Seth, and after Seth had shown that he could easily subdue her, she had begun making plans to flee from him or report the problem to Rose, but then an idea had popped into her mind, a ruthless, devious, and most especially beneficial idea.
Immediately it came, Leah held onto it, and within a few seconds of thinking, she had drawn up a plan and then immediately moved to carry it out.
Though Leah didn't know much about Seth, knowing he was a vampire and a perverted one to boot was all she needed to scheme against Seth.
With a few right words to Seth, Leah began moving as she planned.
Though there was a chance that Seth would see through her actions, that didn't worry her as in the end he lacked a very important piece of information.