Fascinating...
Noddies are a type of b-roll - not stock footage, but not the main story, either. When you see a camera cut to an interviewer while the interviewee is still speaking, you'll often see the reporter nodding along - thus the name. It also includes smiles, sympathetic noises, cocked eyebrows, or just about any other reaction shot intercut with the main story.
Please, do continue...
Crews will need to record the interviewer making a range of expressions on the same day, on the same set, so that they’re not wearing different clothing in a different room if they haven’t filmed on the day, and so that they’re not making the wrong reaction, such as smiling during emotionally charged stories. These may be shot while the interview subject is in the studio, but it's just as frequent, if only one camera is available, to shoot them when the subject is already long gone from the studio. They also serve to help the audience feel engaged in the conversation, and more practically, to cover the editing, as the more abrupt cuts would be obvious without noddies to cut to.
In this 1998 interview of David Bowie, the camera stays on Bowie answering a question before briefly cutting back to Alan Yentob in a noddy, meant to indicate that he’s engaged with what the interviewee is saying.
