What does it mean to know a person? In his famous article, McAdams (1995) addresses this question from the perspective of personality psychology and concludes that personality traits are “the psychology of the stranger.” To really know someone, you need to know more than just how they typically think, feel, and behave on average (a common definition of traits). You need to know how their thoughts, feelings, and behaviors change depending on their role and context, why those fluctuations occur (the underlying motives and causes of those patterns), and how they make sense of their own patterns over time (their life narrative). In this essay, we argue that although there has been little empirical work on within-person fluctuations in personality, the time is ripe to examine these patterns. New technology has made it possible to quantify momentary thoughts, feelings, and behaviors, and to track the contextual factors that underlie these fluctuations (i.e., “personality signatures”). By capturing individual differences at this dynamic level, we can gain a better understanding of how people differ from one another. This will also open the door to new research questions, such as investigating the amount of insight people have into their own and others' personality signatures.