Most high fashion designers take their clothing line seriously and typically require their models to not smile while walking down the runway. The idea is that the audience will look solely at the fashion designs and not concentrate on the model’s face or personality. In other words, the models become walking mannequins. The exception would be a designer like Betsy Johnson! Her fashions are all about having fun. In fact, Betsy does her famous cartwheel after every runway show!
Commercial print modeling is all about selling a product and most models are required to smile and look friendly. Your local department store runway shows are more relaxed and you will see more smiling models working the runway.
If you are booked for a runway show the designer and/or choreographer will tell you precisely what type of facial expressions that want you to project.