We would assume all senior politicians have fairly extensive media training and one thing they would be told is to be polite. Fairly obvious you would think but it can go too far and politeness can tip over in to smarminess. So be warned!
The Health Secretary Jeremy Hunt used to have a habit of addressing his interviewer with a lengthy and deliberate “Good morning/afternoon/evening” whether the interviewer had started the interview with a welcome or not. He also always mentioned the interviewer’s name, so you’d get a protracted “Good morning Bill, Good morning Susanna” when the interviewer had just asked him a direct question. In my opinion it made Mr Hunt a) look like he was picking up on the interviewer’s lack of politeness b) look like he was stalling and playing for time and c) come across as unctuous and ingratiating. All from six words at the start of the interview before he’d even got on to the subject he was there to talk about. Not a great start!
The best rule of thumb is if the interviewer welcomes you then return the greeting, quickly and succinctly because you want to show you’re eager to talk about the subject in hand. If they don’t welcome you and just ask you a direct question then just get on with the interview, you can show courtesy and politeness in many other ways during the interview, you don’t have to rely on the initial greeting to prove you’re a nice, well brought up person!
Oh and don’t use the interviewer’s name, it can look like you’re being overfamiliar and it’s a dangerous game to play because you run the risk of getting it wrong!