This dissertation confronts a problem: what does it mean to act out of character? I argue that writers of the long eighteenth century such as David Hume, Samuel Richardson, Choderlos de Laclos, William Godwin, and Jane Austen used this question as a heuristic for rethinking models of identity, agency, and accountability generated by the debate—ubiquitous in the period—about free will. Within the terms of this debate, persons were either autonomous psychological beings or faceless functions in a law-bound universe. While libertarians insisted on the importance of desire and intention, their opponents believed that a necessary connection between character and action was the only possible foundation for moral judgment. Writers working at the intersection of philosophy and literature were especially alive to the limitations of these two paradigms as they struggled to reconcile theories of agency with representations of individual experience. This project argues that such authors understood “one-offs”—fleeting passions, rash actions, accidents—not as exceptions, but as examples from which to formulate alternative accounts of human behavior. These thinkers showed that it was impossible to define either character or action based solely on the relationship between them. Each chapter of “In and Out of Character” outlines a distinct set of terms designed to make sense of the problem of acting out of character. The first two chapters show how passions can weaken or eclipse the causal link between character and action. In Hume’s Treatise, for example, a pathologically excessive sympathy can cause a person to identify more with someone else than with herself such that she no longer recognizes her character and actions as her own. Chapter One argues that for Hume, only the passion of humility can rescue the self from being swallowed up by sympathy. Chapter Two shows that in Richardson’s Clarissa, passions like pride and shame circulate unchecked. As a result, a person can become affectively and morally connected to an action that is not, properly speaking, her own. In Clarissa, it makes no sense to talk about individual agency or individual responsibility. The affective and moral consequences of actions are always shared, and there is no way to punish a guilty person without hurting the innocent people around her. The last two chapters of the dissertation shift the focus from passions to actions. Chapter Three’s analysis of Laclos’s Les Liaisons dangereuses reveals that because actions can unfold in a number of ways—via different causal chains—a person may perform an action that is in character, but in a way that is out of character. Chapter Four demonstrates that in Godwin’s St. Leon, characters only exist in the third person. Godwin imagines a world in which only other people are agents, and by extension, only they are candidates for moral approbation. The self counts for nothing—is nothing—and in its absence, true justice can emerge. The dissertation ends with a brief Coda that focuses on a particular case of acting out of character in Austen’s Persuasion. I argue that while eighteenth-century authors use the idea of acting out of character to discover what character is and how it should be depicted, Austen uses it to gesture towards the formal and ethical limits of the realist novel.