Because you're the only usability specialist in your company, you're involved in all stages of the product development process. You might actually never get to participate in all aspects in one day, but here we'll suspend that bit of reality so you can get a better idea of what the career is like.
You work for a medical device manufacturer that wants to develop a next-generation surgery tool called a laparoscopic laser. You attend a meeting with the CEO and representatives from marketing and finance, who are all debating the product's rough parameters. While you make suggestions and raise questions, for the most part you're a listener. You leave the room with a list of musts, maybes, and questions about the prospective product.
Next, you read up on the current generation of laparoscopic lasers and then observe three surgeons who are using them. You ask questions and take notes about what they like and dislike about it, and how they suggest it should be improved.
You write a report summarizing what you've learned. Then, engineers develop a prototype of the product that comes closest to meeting both the company's and the surgeons' desires.
You recruit and observe surgeons to use the prototype, again asking questions. You make recommendations for changes in the laser. The final product ends up incorporating only some of what you had hoped for, but you still feel a sense of pride for having helped ensure that the new laser will be more effective and pleasurable to use.