Interest (emotion) is a feeling or emotion that causes attention to focus on an object or an event or a process. In contemporary psychology of interest  it is used as a general concept which encompasses other more specific emotion terms, such as curiosity and to a certain degree surprise, in a similar way the general term anger encompasses other terms for the emotion such as rage (intense anger).
The facial expression of emotion of interest shares most of the features with surprise:
- Eyebrows that are raised so they become curved and high.
- Stretched skin below the eyebrows.
- Horizontal wrinkles across the forehead.
- Open eyelids -- the upper lid is raised and the lower lid is drawn down, often exposing the white sclera above and below the iris.
- Dropped jaw so that the lips and teeth are parted, with no tension around the mouth.
However, the facial expression of interest encompasses additional features which are not characteristic for surprise, such as:
- Dilated pupils.
- Widened nostriles.
- Visible tongue -- in slightly upward position (while, for example, in disgust the tongue is visible in more or less downward position)
- http://metainterest.googlepages.com/tenet_1 A theory of different stages of interest (from noticing something, wondering about it, being curious, to being fascinated, astonished, and, in ecstasy)
- Silvia, Paul (2006) Exploring the Psychology of Interest. University of Oxford