During his recent appearance on The Graham Norton Show, Sir Ian McKellen recounted a story involving Dame Maggie Smith at the 2002 Oscars. That year, he had been nominated for Best Supporting Actor for his role as Gandalf in the 2001 film, The Lord of the Rings: The Fellowship of the Ring.
As Norton was asking - and McKellan was anticipating his question (seen during the 0:09 - 0:10 portion of the video), the actor displayed a particular facial expression known as a "Lateral Lip Purse". Although extremely common, it's very rarely discussed in the nonverbal literature.
It's crucial to note that the remainder of his face is relaxed and neutral. For if there are other nonverbal signals accompanying the lateral lip purse - especially involving the eyes/eyelids - it has an entirely different meaning.
Moreover, be careful not to confuse this with a "Forward Lip Purse" (as the name suggests, with the lips protruding forward, rather to one side) - as it too projects a very distinctive and different set of thought-emotions.
This example of a lateral lip purse is quite classic, however, the camera angle does not afford an optimal view. If McKellan were looking straight at the camera it would have been better for teaching purposes - yet after careful observation one can discern his lips are vectored strongly to his right.
A lateral lip purse (with the remainder of the face neutral) is a signal of open, non-secretive decision making. The easiest place to see this expression is at a restaurant - while the patrons are perusing their menus'.
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