Hello!
I guess you refer to a normal case (vs. a written case) and in concrete, to after the issue tree is layed out.
You need to use both of them, the info of the case + your personal toolkit. For this, to enrich the last one as much as possible, the best way is to practice this sort of cases.
At the end, someone who master cases is able to leverage a rich toolkit (built by doing many prior examples and getting good feedback) and adapt it very well to the specifics of the case. For this it´s always said that "general" approaches don´t work, and as an interviewer it´s very simple to see who is giving you a solution learned by heart or extrapolated.
Hope it helps!
Cheers,
Clara