Interest is a feeling or emotion that causes attention to focus on an object, event, or process. In contemporary psychology of interest, the term is used as a general concept that may encompass other more specific psychological terms, such as curiosity and to a much lesser degree surprise.
The emotion of interest does have its own facial expression, of which the most prominent component is having dilated pupils.
See also 
- The following can be read in a review or blurb concerning Paul Silvia's Exploring the Psychology of interest: "Anyone interested in emotions will find this book on the emotion of interest immensely interesting! If you are among those who question the status of interest as an emotion, this book will convince you. This very real emotion not only exists, but also plays a major role in shaping our lives. This book goes a long way toward documenting what I have long believed. Of all the emotions, interest has the greatest long-term impact across the life span."--Carroll E. Izard, PhD, Trustees Distinguished Professor, Department of Psychology, University of Delaware
- Silvia, Paul (2006) Exploring the Psychology of Interest. University of Oxford
- "We cannot help but reveal our interest in (and attraction to) others through the size of our pupils."--Satoshi Kanazawa, PhD, an evolutionary psychologist, Reader in Management at the London School of Economics and Political Science, and Honorary Research Fellow in the Department of Psychology at University College London, and in the Department of Psychology at Birkbeck College, University of London, in his blog Scientific Fundamentalist:
- A theory of different stages of interest (from noticing something, wondering about it, being curious, to being fascinated, astonished, and, in ecstasy)