It's not hard to understand why. Most advertising shots have very high production values. Along with the photographer there will most likely be an assistant or two helping set up lights and reflectors, a producer making sure their is access to the location and getting everyone co-ordinated to turn up on time, a stylist looking after the wardrobe, a make up artist, and finally the art director to make sure the shoot matches their original idea.
With all that going on around them while they try to follow direction from the photographer, its not hard to understand why the models, unless they are very experienced, tend to look a little wooden in front of the camera. Here are a few tips that can help:
1) Give them a story
Be sure to communicate with your models and give them a scenario to work with. What have they just been doing? What are they feeling? What should they be doing next? How about they've just come to the end of their afternoon run in the summer heat and need to cool off in the fountain?
2) Put down the camera
One way to guarantee static photos is to accurately place the models in position and then shoot. Instead, put the camera down and try getting them to act through the moment. Look for the little things that make the scene natural - a facial expression, or a body position. Often it is the 'in between' moments that look most casual. Here the models just started kicking water at each other - it wasn't anything either of us had planned.
3) Shoot, shoot and then shoot some more
Now that you know what it is you want to capture, get the models to work through their vignette over and over again until you capture exactly the moment you want. Turn on the motor drive and just hold down that shutter button. You'll have 500 frames to edit, but one of them will have that spark and capture the snapshot feel you're looking for.
So there it is: the more natural you want the scene to look, the harder you have to work for it.