It is generally recognized that finding your customers and engaging in meaningful conversations with them is easier once you have a shipped product no matter what the market culture is. However, putting a process in place to talk to your customer (a.k.a internal interviewees) is always beneficial.
Finding the right candidates that match your identified target groups when you are in the ideation phase of your startup is one of the operations that can be extremely painful and subject to strong bias if not driven by a substantial level of rationality and why not objectivity.
Customer segmentation is the practice of dividing a customer base into groups of individuals that are similar in specific ways relevant to marketing, such as age, gender, interests, and spending habits. (taken from salesforce.com)