Preparing for a interview as a Beauty Advisor depends on the counter you have applied for. I worked in the Estée Lauder counter, so I prepared by learning about their top selling items and their Company philosophy. Learn about the brand you want to work for. You don’t really need to know every product, or anything specific but it will help you stand apart. Most importantly just have a friendly outgoing personality, and prove you have sale and communication skills.
Show up in your best skilled makeup, for instance a cut crease, to show that you are talented and deserve a spot. Wear professional dress as well as prepping for how you would handle a sales position, and how communication would work in this job for you.