The biggest difference between amateur, all-volunteer fashion shows and the big time is the way the models walk the catwalk.
In an amateur show (and I exclude fetish and goth "fashion shows" from this because they are more often based on flash and fun than fashion), the models will walk as they normally walk around on the sidewalk, with perhaps a lift of the chin or an arch of the back because they know it makes them prettier (mom tells us these things). To the casual observer, they will be missing something intangible. I work occasionally as a fashion model (mostly for fun, occasionally for pay, but I'm too short to ever be a pro), and it took me years to figure out what I wasn't doing.
Thanks to America's Next Top Model, more people know what I'm talking about now than they would have years ago. There is more to modeling than just being pretty and walking around.
In essence, the Model Walk involves lifting the foot enough to clear the ankle while keeping it close enough to brush the opposite leg(and with models being as thin as they are, this can take effort), fully extending during the forward step, and landing the foot slightly across the median between the two feet. A freeze frame would show (and often does, if you look at fashion show photos) the legs crossed over one another when both feet are on the ground, and a slight 'pigeon-toed' effect. Side-to-side movement is concentrated almost entirely in the hips, and the shoulders are kept down and mostly stationary in the style of ballet dance. This must be done while balancing on often ridiculously tall or skinny shoes. High heels help with posture, and posture helps with high heels. The face is kept exactly level and aimed forward. Expression should be almost loose, but the jaw should be shut while the mouth and eyelids are slightly relaxed. Blink as little as possible while still keeping a serene or blank expression. Smiling is usually up to the designer or director. At the end of the catwalk, a pose is expected and can be any of a number of things, generally with a cock of the hip and/or shoulder, or a flourish to demonstrate a feature of a particular item of clothing or accessory. Sometimes layers are added or removed. The model spends maybe two seconds at the end of the catwalk, then turns and heads back to the 'curtain'. Often the next model will have already started her walk, with the two meeting halfway.
Why is this customary? Several reasons. Firstly, fashion shows are infested with photographers and videographers, and the photos and tape will be published in all manner of media. A model cannot afford to ever look anything but composed and graceful, because a photographer will likely capture her faux pas and it can damage her reputation. This method of walking minimizes the time in which she can look awkward, off-balance, or short-legged. The immaculate posture makes the clothes look good and keeps her literally on her toes so she can walk in the heels. Expressions should be neutral so as not to draw attention from the outfit to her face (this is not always the case; makeup is sometimes a larger part of the presentation). Having the legs crossed at the ankles creates a tapering, symmetrical presentation that is pleasing and feminine.
I was unable to find the origins of this walking style in my research, and this node is based almost entirely on personal observation and experience. Further information submitted from noders is requested, and will be added when received. As a side note, "model walking" is a good form of excercise. It uses the thighs, while most kinds of walking do not. A slightly toned-down version of it, used in public at various events, will assist in making you look graceful and help you balance on your high heels. Give it a shot. It's fun to play dress up, and walking the walk is a part of it.