Hi, I was a model for many years. For some really silly reason. The designers that allow you to wear their designs, believe that if the model smiles on stage, it will make the audience focus on the model and not the design.
We are told backstage, to keep a straight face at all times!
Very rarely is there an exception to that rule.
If the designs are more light hearted - bathers, sleep wear etc, the designer may request you smile, to give the appearance of fun.
Hope this helps a bit? Skellie