The stimulus can be either, although its usually some type of quote or something (check the sample hsc papers). But for assessments it really depends on teachers, either way you should be prepared for both.
The reflection statement consists of stylistic devices you have used/borrowed from the prescribed text within your discursive, imaginative or persuasive essay.
It should have a introduction and conclusion. The paragraph distribution depends on the number of stylistic devices used. If you want to get a really high mark, try to have like 1 paragraph for each stylistic devices and discuss how you effectively used this device from your prescribed and its effects/purpose etc.