Rob,I understand your problem perfectly!! I do a lot of interviews and one of the things to remember is the camera is the greatest modifier of human behaviour.So we must take this into account right up front with most people.I find its essential to make a connection with the person being interviewed if they look or sound nervous in particular.Often its a smile,a light-hearted remark,or finding out if you have friends in common.This is often not possible with the time constraints of today's world.But there would be very few people who don't appreciate your effort to make them feel more comfortable.In doing a major interview it is important to sit down with them beforehand and listen to them,have a cup of coffee,and make them more at ease.Let them realise that they can trust you.Their voice levels will tend to rise as they talk more, especially if its about something they are familiar with.
I prefer to use either a Schoeps(MK41 with GVS swivel and a PSC small universal mount) or a DPA 4071 lav for i/vs.The size of the mic matters for most people as the least intrusive is the best.
Close all the doors, windows and any where there is an air gap,as unwanted sound will get through.I try and control the background early on before the i/v is even set up.The less fuss the better during the i/v.
Hope this helps.
Brian