Most of the models I work with have big personalities and only need a little suggestion and direction to get moving, but even with these models I often give examples of what I'm looking for and I always point out and ask them to pause and repeat something they may have done spontaneously that catches my eye or works with my intent. When you work with subjects that don't traditionally consider themselves to be models or maybe a little shy is when the photographers direction becomes very important. Creating a comfortable condition and offering lots of direction and possible poses, ideas, or feelings seems to help out in those situations. For really shy and uncomfortable subjects the best way to get the shot is to just catch them with their guard down while they aren't focusing on themselves in front of the lens.
Hope that helps : )

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