Here is the tricky part where it becomes somewhat counterintuitive: even when you are talking about a serious and depressing subject such as a crisis or a massive flood, should you be smiling? Wouldn’t you look like a massive, uncaring idiot?
Here is the thing about television: if you have just a little bit of a smile, it will not just look like you are smiling but you will simply look more relaxed. You will appear more comfortable and confident. Now this is especially true when you are being introduced with the cameras on you but you are not speaking. The natural thing to do is have your face go blank and when your face is blank on TV, you will look bored, boring, flat and uninterested and needless to explain, that is not a good look for most people.
What I recommend is just having a slight smile on your face. You do not have to show off your teeth or flash a gigantic toothy grin at the cameras- you are not a Tv evangelist. But just a little bit of an upturn can make you look much more authoritative, confident, and believable regardless of whether it is good or bad news story.