Which factors determine whether academics get paid when they give interviews in the media (or other kind of media appearances, e.g. debates)?
I am mostly interested in the field of computer science, and English-speaking venues.
I am also interested any statistics giving some insights on how frequently academics get paid for media appearances, and how much.
Generally speaking, academics do not get paid to be interviewed by the mainstream media. The interview is often regarded as a chance to promote one's research, expertise and increase one's reputation and fame. Also ego may play a role (which might be exploited by the media).
Things might be different when one already has received a fair amount of exposure in the mainstream media, and is regarded and recognized as an expert on a certain matter. In those cases you might even be in the rolodex of the media agencies and journalists (which leads them to you when anything pops up in your domain). Also it makes sense to have an agent represent you when you have reached this status, because the agent can negotiate the payments and relief you from the communication overhead. But for the vast majority of the academics that is not the case.