Documentary profiles are difficult, that's why finding the best possible subject is crucial.
A good documentary is able to find a story in the mundane aspects of everyday life. It can look at an ordinary person and find a dramatic character that speaks to ideas and conditions far beyond their own experience. This can be to highlight a larger social issue, archetypal paradigms, or mimic the hero’s journey in some way. If you are intending on producing a non-fiction film that focuses on a person as its central piece then you have to make sure that you find the right subject.
The first, and most important thing, to consider is access. If a subject is not willing to let you into their life in an intimate way then the project will not work. Since you are going toward a profile in your project they need to be on board in a complete way. This includes giving you access to them personally and professionally, including repeated interviews. Many people will be willing until it becomes very personal or the project begins to challenge them. Make sure they are aware of your intentions and what it will take on their side.
You are going to need to find someone whose life, and character, is representative of a larger idea or perspective. All great films look at something small as a way of discussing something on a larger scale. This does not mean that they need to be a stereotype or caricature, but instead someone with open contradictions and passions. Don’t decide exactly what the perspective of the film is ahead of time, but allow the person to be a driving force that will eventually illuminate something more profound.
Their life, especially at the time of filming, is very important because you will need to be able to construct a story arc. If they are just sitting idle then you will not be able to actually create a plot of sorts. Make sure that in telling their story you are able to translate a structure to the film itself.
Their involvement with other people, possibly in the public sphere, is also important. If there is no one else to discuss that person, or the ideas and issues they are raising, then they will really be the only person talking. There has to be outside perspectives to give this a three-dimensional shape so it is important that they are able to give you other people to talk to. Even if they do not share this with you it is important to have the ability to find other people, possibly through their own professional or personal life.
What you have to consider here is why this person is important. What does their story really have to offer? If you can answer that question easily then you should have a good project in front of you.