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in extreme circumstances, throw one of your shoes, just be sure your aim isn't too good : )
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I've used a slightly different variation of this idea quite successfully. When working with a new model (or one who has found it difficult to express different looks & emotions), I introduce her to "the fish".
The fish is a small, stuffed animal approximately 5 inches long which sits on the strap of my tripod. If I notice the model is freezing up, I'll gently remind her of "the fish", and move it to my knee (if sitting) or put it in my pocket (if standing). This usually generates a smile, or a "questioning" look, or possibly a look of disbelief ("you wouldn't REALLY throw that at me, WOULD YOU?").
After that, it's three strikes, you're "fished"... I gently toss the fish in the general direction of the model, my finger at the ready on the shutter button. I almost always miss, but when I don't (which really IS a miss!), the stuffed animal lands in her lap or bounces off her legs. After that point, I usually only need to "remind" her of the fish to get a new look or emotion.
I wouldn't suggest this for a "Pro" or experienced model, but with new models, it's been extremely successful for me. YMMV.