If you are designing a product for a process that may have a variety of users and protocols when using the product, you will never be able to see the scope of that variation by doing site visits alone. This is when a focus group of subject matter experts may help you validate, while further defining, and describing the processes that will have the most predictable and successful results with the product.
This kind of focus group will generate the best results when you do some upfront planning and make sure you have a support team with you when facilitating.
FOCUS GROUP PLANNING
FOCUS GROUP PREPARATION
FOCUS GROUP FACILITATION
The most important factor in getting good data from a focus group is having a facilitator the understands and acheives the objectives of the session. The facilitator must also be very good at introducing the agenda segments and facilitating the conversation to keep it moving, keep it on track and to engage all attendeees.
If the process and product are very complex, I have heard of having multiple notetakers that focus their attention on one or two of attendees. This way none of the conversation is missed. I haven't tried this yet, but I intend to soon.