UJM is a user-centered methodology. No wonder, that you need to research, define and select your users carefully before the workshops. In many cases, we don't have access to real users' data or interviews. That's why we are working with User Personas. A UP is your target group personalized and merged into one persona. A User Persona is a set of characteristics typical for one of your users' types.
You always need to interview users or people representing the different User Personas before you'll start the workshops. And it's always a good idea to have real users attend the User Journey Workshops.
It's a good idea to have two separate interviews with a representative of your critical User Personas. During the first, you can define the value proposition, and during the second, you can negotiate the best solutions with the user and sketch our the scenario that you will follow while building the UJM.
You start by defining the most important actor (User Persona), based on your clients' business goals, and find the answer to the questions.
From our experience, you don't need the most basic demographic information on the UP, such as age, sex, residence. Focus on their characteristics, attributes, motivations and their background.
If you have more than one UPs, attach some percentage weight labels to them, such as 10%, 50%, 40%, so you always keep in mind their importance.
Also, you need to know that the actors (UPs) are not only customers. You have to remember that on the other side of your digital product, there are members of your clients' organization, who often use it through a dashboard - that's why you also need to hear them out.
Also, try to figure out what are the key actor's goals? Do they align with your clients business goals? That's the essence of your clients' business model and the UX design. Usually, these goals are totally different, but it's good to check and ask. For example: Would you trade your data in order to have this service for free?