I am mainly with Juan Lanus on this but, wanted to make some slightly different points/download.
I think one to one interviews are best, and a semi formal interview is the best format. You need to realise you will always have the 'power' and part of your job is to allow the person to have control in the conversation. This is different to many other types of interview situations. This way they will be more likely to talk about things that make them feel stupid.
Behavioural interviewing techniques are really important, talk about what they have done in the past not what they may do, everyone has good intentions but rarely act on them. This will ensure that you get realistic data on their activity. Also open questions et al
Start the conversation about neutral subjects, most people will be guarded as they will not fully understand why you are doing what you are doing. Then slip into the more personal, starting with loves and hates is usually an 'easy' way in.
Find the thing they are passionate about, get them talking about, people love talking about the thing they love. Get them talking it does not have to be about work.
Listen to what they are saying and ask questions about what they are saying not what you have on your list. I have so often heard people miss an interesting bit of information because they are more focussed on the list of questions than the person.
Have a list of questions you can fall back on if you get stuck. Do as many as you can and always listen back to your recordings and work out where you could do better.
Remember you are not finding out about their use, you are finding out about them.