One common buy-in is from one or more people who are intimately affected by the problem, typically meaning that the symptoms of the problem hurt them, or else they are forced to have to deal with those symptoms in order to do their own job properly.
These people are often desperate for a solution but they have a keen sense of what the problem is, they are frustrated by the reality that no-one seems to have listened to them in the past (or else surely the problem would have gone away), and they are skeptical that anyone else's solution will really work.
So the "twist" on the basic buy-in is to make sure they see and feel (at an emotional level) the connection between the core problem (conflict) and their own own, very specific issues, to a depth that is unequivocal.
For people who are very much involved in the subject matter (they are a part of it and/or, very much affected by the UDEs) - but they will in no way feel responsible for creating the problem (therefore no defensiveness).
1. Before your meeting, expand the CRT to show the symptoms that very much affect the person, if the starting symptoms don't already do that.
2. Read through the Current Reality Tree from the bottom upwards, to gain a buy-in for the core problem and precisely how it is responsible for a myriad of negative effects (symptoms or, in TOC jargon, UnDesirable Effects, UDEs). Show clearly how the Core Problem leads to their UDEs.
3. Expose the Cloud. This eliminates the finger-pointing because you are showing that the core problem is the result of a genuine dilemma that looks to be unresolvable. Explain the key assumption under an arrow, and the Injection you are proposing that will invalidate the Cloud and break the conflict.
4. Expose the Future Reality Tree, from the bottom-up. Let the person improve on it as much as possible. Be explicit as to how every one of the UDE's that effects him is clearly dealt with. Trace every suggested negative side effect to it's unpleasant and unwanted conclusion, and deal with it together if necessary i.e. sometimes you can come up with the "Injections" to the plan that will prevent the negative side effect from emerging - but it's more powerful if THEY do that for themselves. The more of their ink on the page, the stronger their ownership.