I'm not really the best qualified person to reply but since no one else has I'll tell you what I can....
There are sample questions to let you know the sort of thing that might be asked in Appendix 1 of
The Teaching Diploma Syllabus.
The viva questions, if it's anything like the questions asked in the performing ones, will be asked directly and you will be expected to talk about them ideally without further prompting (although you can, of course, ask for clarification of the question without being penalised). If you are not getting to the points that the examiners want they will prompt you with further questions, although if these prompts are needed for important points the marks will probably be lower.
Here is an example of prompting in the performance viva (piano) from
someone posting their viva questions a while ago...
QUOTE
1. What is the nocturne ? (free, improvisatory piece coming from the Romantic period.. ABA form.. written before only by John Field)
2. What do you mean by ABA form? (describe basically the difference of parts, how they contrast, possibly modulate to other key, then come back)
3. Is it more a day or a night piece? (its a NIGHT piece!! but i didnt know that, ahh!!!! so i was sort of ambiguous.. and he sensed it, so he was like "is it more for the evening?" so i said yes.. but i messed up there. nocturne means night, apparently, in some foreign language!!!)
As you can see sometimes they'll prompt because they want you to expand on what you've said, I doubt that they will take off marks for that, but sometimes if there's a piece of information that they want to get out of you they will prompt for it.