Like Ted Ehara, I think you have to have some kind of 'script', which you revise and polish as you progress. Of course, the examiner is usually aware of this as well, since he/she has gone before.
Last summer I held a 4th dan grading test. I consulted a variety of material given to me by senior Aikikai shihans and came up with this:
(1) The candidate had to teach a class, with the examiners following him around with a note pad. Explanations as to techniques taught, sequencing, explanations given in class etc, were also required.
(2) The test itself required the candidate to demonstrate the level of technique and awareness that I considered appropriate for 4th dan:
(a) suwari-waza with one uke, with the examiner calling out the attacks and techniques required;
(b) hanmi-handachi with 1 uke, again with the examiner calling out the attacks and techniques required;
(c) tachi-waza with 2 uke attacking in succession, one after the other, with the examniners calling out the attacks required;
(d) free koshi-waza and or ganseki (2 uke attacking in succession);
(e) Tachi, jo, tanto dori with 3 different uke in succession, i.e., in a circle of three, with tori moving across the circle after each technique;
(f) jiyu-waza against 4 uke attacking simulataneously.
All uke were chosen by the examiners, never by the candidate.
The candidate, who passed, told me later that he was exhausted almost to breaking point, and at times the test was an almost mindless production, of course, of the 'script' he had learned.
Best regards to all,