I think if you are supposed to only "interview" this person, this means you are going to direct this person to another professional. I think, in this case, you can inform the person about the procedures briefly and that you will be sharing information with the other colleague, if the person consents, then there is no problem. But I am not sure this is what you mean; I think you would help clarifying the question by adding specific details.
Regardless of the persons state the problem is not attached to them as closely as the western mental health model does, ie the person is the problem. Rather the problem, unhelpful reactions and antisocial behavior for example, is separated from the person and put into the middle of the room. Then everyone who is affected by the 'problem' including the sufferer themselves, family, friends, flatmates, police, employers and last of all medical people all have an equal say in how the problem may have come about and what is going to be put into place to bring about a resolution. There are no secret conversations and health professionals express their feelings, fears and hopes out aloud in the presence of all involved. This increases insight, honesty, resolution and teamwork, the patient/client/sufferer being the most important member of the team.
Open dialogue experiences unusually high success and many 'sufferers' go on to be actively involved with helping others overcome their own mental health episodes of distress. My question is if this is so successful why is it not happening in other parts of the world? Every one has first and foremost the right to human, to be unwell and not coping at times, to stress out, burn out and emotionally fall over. So approach with respect and gratitude that you may experience emotional distress as a second hand experience and not first hand..... next time might be your own if life does not treat you well.
In my opinion the best way to start an interview of any people (with or not mental illnes) is saying: "hello, i am Felipe"... introduce myself and presenting the intention to know... The first question ussually is "who are you?"; next "how are you?" and then... "do you have/need/like to speak something with me?"... some times i comment the weather, news or some situation of the context... Sounds simple.... In my opinion, a naturalistic, transparent and real aproximation is the best way (for me)... All the people have (and need) a personal approach to interview, that is the "art" of the contact.
Share feelings, be honest and respectful is good and necessary for me... be a person with another person like wrote Rogers.
Like Felipe, I approach an encounter with a new client as a conversation, rather than an interview. I introduce myself and tell them why we are having the conversation. My task is to facilitate the conversation in a way that is therapeutic and elicits the necessary information.
I'm not sure about the context for this interview. Is this about interviewing them as a research participant? If so then you approach the interview as if it is any other interview. Can you clarify why You are interviewing them please?