Whether for professional purposes or for fun any discussion between two people could be viewed as an informal interview in which opinion and/or information is exchanged.
Interviews are not only a formal discussion between a journalist and a celebrity; it can also be a chat between two friends, or two colleagues.
What matters is the information contained in such interviews and how valuable it is for the public.
Obviously interviewing has many forms, it can be on TV, in a newspaper or magazine, and it can also be online! With the unprecedented success of the internet and the adoption of the web, there is no reason that interviews can't be done online.
Actually, informal interviews have been practiced since the early days of internet through the chatting services.
The only difference from the formal ones, other than the style of course, is the persistence of the interview.
While the latter are published and made available to virtually anyone to read, the former was mostly confined to the private domain.
There's no doubt that chatting is a form of interview.
Even if it does not involve a well-known celebrity or an experienced journalist, nevertheless it still can contain valuable information that could benefit others.
Consider an example where one is asking his colleague about technical matter related to a certain project.
Sometimes this exchange is done via email but there are other times when it is done over a chatting service like MSN.
The best thing to do at the end is to publish this discussion that can be also thought as an interview.
Even though there are no nationally or internationally known figures involved, however the contributors are valuable to their colleagues and other project members, and their issue they discussed is certainly important to them.