How do you respond if the banker asks deep technical questions about a deal detail such as multiple, synergies, or tax structure?

Answer what you know clearly, distinguish public facts from confidential details, and do not bluff. If you do not know, explain how you would think about it. Example: for synergies, discuss cost versus revenue, phasing, tax effect, and cost-to-achieve. The interviewer prefers honest structured reasoning over invented precision.