Build Rough & Ready, then Refine it
As quickly as possible build a rough and ready simulation (it doesn't matter if some data is missing or if it does not run properly) and take it to show your client. Experience has shown that even static drawings on a computer screen can do good job of getting your client to understand
more about what it is they will get from you and
what aspects of the conversation, have been misunderstood and
what aspect of their worries are different now that a consultant has put some structure on them.
This phase is much more important than it may seem. Experience is that the consultant's first idea of what simulation is appropriate is never much like the simulation at the 'end' of the study. This is not because the consultant was necessarily wrong, but because the client's needs evolve throughout the process.
It doesn't matter how rough and ready the simulation was because you now have to start again!