The answer is 'not too much'. This is particularly true where the process includes a knowledge 'recipient'. Having an in-depth understanding of the subject-matter can be a significant limiting factor for a facilitator and at worst, reinforce commonly held misconceptions. An peer-expert conducting such an interview might be significantly inhibited in...
- Being able to ask the 'dumb fool questions'
- Asking for clarification or for examples to illustrate a point
- Recognising and testing assumptions, cultural or organisational norms
- Demystifying acronyms
- Questioning political expediency
All of these go towards validation of the knowledge being provided - a vital part of the process of knowledge transfer. Any resulting 'knowledge asset' should not make assumptions, for example that outputs that are generally accessible will only be used by someone who is already an expert.
Pamela Hinds of Stanford University describes experts' cognitive handicap as 'the curse of expertise'. In referring to education, the most obvious form of knowledge transfer, psychologist Susan Birch of the University of British Columbia in Vancouver says "to teach effectively, you need to see things from the naive perspective of the pupil - and the more knowledge you have acquired, the harder it becomes".
Of course it is important that the facilitator does their homework and has a good understanding of the topic area, the nature and scope of the work and a 'heads up' of any big issues to be explored later. Important avenues of enquiry or 'difficult' topics could be missed or avoided without this prior understanding. A skillful facilitator can recognize fruitful avenues of enquiry and probe for detail, without having a detailed understanding of the topic.
How much understanding? Just enough.