AGGRESSION IS A BAD THING
The police will always turn it against you - you bullied a witness in the hope that you could persuade that witness to perjure him/herself. etc etc..
So you should not introduce yourself and then say "I want to question you about..." That is aggressive.
You should always ask for their help.
Most people like to help others . In particular you should then be "dumb" You do not know much about this case, "but I've been told that you know a lot about it and you would tell me."
Always let the interviewee think that they are in charge of what is going on. Demean yourself. If necessary, ask if you can use their toilet - that always puts you in your place.
Do not introduce evidence you have got from the files. It is always "But somebody me that.. " or "Didn't I read in the paper that..." And in working with the evidence they give you, never put two and two together too quickly - work it out slowly, so slowly that they get to the conclusion ahead of you
Let any theory or line of thought be their idea, not yours. Let them be in charge of the conversation - and switch subjects in the direction you want them to go by being particularly dumb, tell them you don't understand a certain point -which is in fact the next subject you want them to talk about.
As Charles Dickens's Uriah Heep said - be humble.