If your interview team has more than two members, you should assign the remaining members the task of being observers. If you only have two team members in the interview, then the role of the observer would also be filled by the note taker.
Of course, everyone on the team needs to act as an observer in some capacity. From when you arrive in the lobby, how you are greeted, whether the work environment is formal or casual, to whether the offices are luxurious or not; these all give you little clues about the company culture. Additional tell tale markers are how the management team interacts within their organization, how they interact with others, and whether there is a strong and formal hierarchy within the organization. All these clues provide insight into your customers and customers' organizational value systems.
The role of the observer is to process the interview in its entirety. They do not need to take notes (unless also assigned that duty in the case of only two members being...