"In general do you think it is better to keep answers short and succinct and allow the interviewer to ask as many questions as possible or to try and talk at length about the things you are confident on?"
Most interviewers will have the skills to drive the interview for you, so you needn't worry too much about it. However:
One-word answers are not a good way of communicating, and will force the interviewer to ask more questions to test understanding. It will also annoy them if they think you are not making the effort to communicate.
An full sentence answer to the question is usually good, with an example if possible.
Don't launch into a 3-minute exposition on everything you know about a subject and stop the interviewer getting a word in. Then it's not an interview, it's a lecture!
Don't throw in other 'facts' to show you know things unless you are prepared to be asked about them. To mention SES or the NOTA or whatever may well seem like a good idea, but if all you know is the headlines then that will very soon be apparent. Interviewers are interested in the limits of your knowledge and understanding, and if you start digging a hole they may well just keep handing you spades with ever longer handles!
Couldn't agree more with the statements above about learning the answers to scenarios. You may get some way on that, but one day you will have to think for yourself. If you don't understand WHY something is done a particular way, then one day you will come unstuck!
By the way... it's GRAMMAR!!