First I make sure that my resume is updated. I print it out really nice on good paper in color. Then I research the company. If it's PR, then what sort of editorial coverage have they gotten lately? What have they done on their Instagram? What does their strategy look like? What is their brand personality? Then just updates with the company, so if they just got a creative director a few years ago, mention that, mention their work. I think it's always important to explain why you personally connect with that brand because you chose this one brand to apply for and to be interviewed by. If you want to work there, what makes you a good match for that brand in particular. Looking at the big picture is something that I like to do before.
Do your research on the company. But at the end of the day, be confident in yourself, don’t stress too much. If it is meant to happen it will happen! Who are you? Why do you want to work here? What's the brand's aesthetic? What are they about? What's their message? Things like that. It's all about presenting yourself and showing the brand that you understand what they're doing and you understand their aesthetic.