Group interviews tell more about the characters then they do about general topic or issues. To do one effectively you need to know when it is right and what methods to use to get the correct type of footage.
The interview tends to be the probing skeleton of any non-fiction film project. With this you ask a person questions that not only shed light on a topic or a situation, but themselves as active participants. With this they are then exposed and seen in human terms, not necessarily with a set motive. This is usually a one on one process as a way of drilling them personally and getting focused responses. Sometimes you may want to see them interact with others in a cohabitational or group dynamic, while remaining as the direct subject of the interview. For this you may choose to go with a group interview.
A Different Style
The group interview is filled with limitations, and it is exactly these limitations that you must be aiming towards if you are going to use it. First it is inevitable that the people are both going to talk to each other as well as interrupt and answer for others. In this way they are not speaking in expository sound bites but instead acting natural in front of the camera. This then makes them as characters rather then what they are saying the focus of the setting. If you are going for expert opinion, such as an author or academic talking about a social issue, then this is inappropriate. If you are interviewing the family of a deceased child then you are going to learn a lot more about their character and family dynamic this way.
Question Their Authority
The group interview is a way of seeing people outside the realm of authority and instead sees them in a more casual light. In investigative news reports when people are put into a group setting, they are often given less respect and seen more as the object of spectacle instead of a valid voice in a debate. This can be a useful tool, though at times dishonest, to transvalue the validity of certain ideas and persons. Make sure that you are aware that anything they say will have less weight in the audience's mind if you are able to see them as a member of a larger group in a single image. Often times the reason for this is that on some level the viewer assumes that if a single person is given their own space it is because they are significant. If they are part of a group then the group is as significant as the single person, and therefore each individual within that group is not.
There are technical problems with this as well, and depending on your visual style you can address them in a number of ways. One of the better ways to do this if you have several cameras is to position three or four different cameras each to look at a different part of the group. One can be positioned on the unofficial or official “leader" of the group, which all group dynamics have. Then the others can be focused on a wide shot of the group or specific faces and bodily movements among the rest of the crowd. No matter how many cameras you have it is important to stay vigilant and change the depth of field and focus ratio often so that you are really responding to them. This is important because they are going to be playing off of each other and the climactic action could happen throughout the entire crowd. Keep an eye on people that are more emotionally volatile in the group and may either inspire a response from someone else or be themselves emotionally sensitive. Show as much as you can in the final cut, especially from the reactions by group members to behavior of other members within the group. This should be the main reason that you are interviewing people as a group, so it is important to keep an eye out for responsive actions.