I am in the process of making redesign proposals for a product. The current interface is quite rich on information and options (think of MS word). I want to get an idea on how important a certain option/information actually is. What would be the right way to find out? I have very easy access to people working with a very similar product to do interviews or evaluations.
Usage data collected by surveying users regarding complicated interfaces with many options is likely to be unreliable. Users can tell you only what they think they do, but that often doesn't match what they actually do.
Consider tooling up the program to collect real-world usage data. This will allow you to rank functions according to usage. Any that are not used at all are candidates for immediate removal. Those that are rarely used may be evaluated further via surveys, interviews, or other methods. Be careful how you ask questions. For instance, you would not want to remove a function that's used "only" once a month and ranked lowly if it is required for a critical monthly task.
In your redesign, you can make the commonly used functions more accessible (on a toolbar or ribbon), while the rarely used functions become less accessible (buried in menus). During the redesign phase, you can do basic (five-user) usability studies to get feedback for iterative design.
If you can, try to directly observe real users using the product in a natural environment. This will give insights into how the product is used that surveys and metrics cannot. Consider the difference between using an EMR with fake patient data in a conference room vs during morning rounds with real patients.
Over time, depending on usage trends, you can reevaluate removal or positioning of various features. Consider how different MS Office is today from two decades ago.
I think the only thing you should do very soon is to interview users who use the product. The deep understanding of the users who use your product strongly inspires you with new insights and ideas. You must interview customers and all stakeholders.
The things that are very good after the interview are to sort out the insights and ideas you get to improve the product and eventually sort the priorities (the business or anything else that matters to you).