Communication with an engineer is only slightly more difficult than communication with the dead.
Lorren 'Rus' Stiles, Sr.
I admit that I don't know everything about subject-matter experts or SMEs (rhymes with please). But I do know that there are some things that you should avoid asking SMEs, the main ones being "Does the user know this already?" and "Do I need to explain this to the user?"
The problem with these questions is that, since SMEs tend to believe that everyone knows as much about technology as they do, they are likely to reply "Yes!" to the first question and "No!" to the second, when it should really be the other way round. Technical writing rule of thumb: No, the user does not know it already, and, yes, you do need to explain it to the user.
Never, never, never let the SMEs tell you how to write the documentation. A SME is the subject matter expert, not the documentation expert (that's you).
Although you are the documentation expert, please do not let this fact go to your head. I say this because some technical writers have given us the reputation of being obnoxious know-it-alls and you don't want to be one of them. If a SME insists that you include something in the documentation and you are sure it doesn't belong there, just smile and thank the SME for the advice! It is not necessary to argue or point out that you are the documentation expert. For one thing, you will offend the SME, a person who has just spent an hour or two patiently answering all your questions; for another, you might just decide on reflection that you should indeed include that bit of information.
It is important that you develop excellent interviewing skills. You must be able to ask numerous questions that the SME might regard as simple, stupid, tiresome, distracting, and time-consuming. In these circumstances, it is difficult for SMEs to perceive you as an expert at anything, let alone documentation, and it can take all the strength of your character to remain unintimidated and friendly under the glare of their possible and perhaps obvious disdain.
I recently had a bit of a brainwave about SMEs. I was writing a document about the Unified Modeling Language (UML) and the Rational Unified Process (RUP) and I was wondering, "What if a newbie technical writer asked me about UML and RUP?" I realized that I would stare blankly at this newbie for a few moments as I tried to formulate my answer. The poor newbie might interpret my blank stare as indicating that I was in the presence of Stupidity personified, whereas in fact I'd be trying to figure out the best place to begin. These musings led to my little brainwave. Maybe all those blank stares I'd received over the years had nothing to do with what the SMEs thought about my intelligence; maybe they were just trying to figure out where to begin answering my questions.
I have noticed a difference in the way that male SMEs and female SMEs answer questions. If a female SME doesn't know the answer to your question, she'll usually say "I don't know." If a male SME doesn't know the answer, he will usually try to provide one anyway. This creates a minor problem because some of those answers will be wrong.
Although I know nothing about psychology, it is my belief that, when a SME gives you a wrong answer, he is not trying to deceive you, but it is possible that he a) really wants to help you b) cannot bear to say the words "I don't know" c) does not wish to appear unknowledgable after you have cast him into the role of "expert" d) ?
I have also noticed that male and female technical writers handle SMEs differently. Whereas female writers generally go to the office of the SME to ask questions, male writers usually request that the SME visit them in theirs. According to Deborah Tannen in her book Talking from 9 to 5, many men feel that "asking for directions, or for any kind of help, puts them in a one-down position." [p.24] I think that walking into a SME's office and asking questions must be similar to stopping the car and asking for directions.
The techniques that you use to interview SMEs come from your own experiences in dealing diplomatically with people. Certainly, your interviewing skills will develop and improve over time.
One thing that used to puzzle me about SMEs is that, unless they are managers, very few of them can tell you how the part they are working on works with the rest of the application or how anybody else's part works with theirs. I have come to the conclusion that the reason for this lack is that each developer usually works on only a small part of an application. It seems to me that asking a developer to tell you how the whole application works is akin to asking a bricklayer to describe how an entire building is constructed. Often the system architect is the only person who can tell you how the whole thing works.