How closely a mock approximates the finished product, from boxes-and-arrows wireframes at the low end to pixel-accurate, fully populated screens at the high end.
Keep the kickoff at low fidelity. If it looks finished, nobody in the room will question the flow.
Fidelity is a dial for the feedback you want, because people critique at the level you finish: show polished pixels in week one and the meeting is about the blue; show wireframes in week ten and the meeting is about your effort. The craft is mismatching on purpose: rough where you still want argument, considered where you want sign-off.