When job searching, one of the first things people generally learn about a position is based upon the job description. This is always a great reference point for the basic guidelines of a job. It details what your duties are/would be in a clear and concise manner. It is the baseline of a performance review.
But what if the formal job description doesn't contain everything that you hope to get out of the job?
This will happen to most people in the course of their career. Job descriptions, while great, are not necessarily the most organic documents. You might be a year or two into the job and feel that you are looking for more responsibility or additional experience within the position. This is natural, but you need to be proactive in obtaining these additional duties. Identify areas in which you seek to gain experience, and approach your supervisor with a plan. If you can detail what you want to take on, and how you feel you can do that, you will have far more success in getting that additional responsibility.
This is advice that goes beyond pure job responsibilities as well. Companies are always looking for people that can take charge and show leadership skills. Doing this will demonstrate your capabilities and enthusiasm. When it comes time for a promotion, the proactive member of the team is generally going to have a better chance than someone who waits for things to happen before acting. Which one do you want to be?