How do you carry out a fit-gap analysis?

I know it's a loaded question, but I would be happy with any advice/ideas to optimize our process.

As an Odoo partner/implementator, how do you avoid nasty surprises late in the process of implementation? How do you make your customer actually tell/show you every part of their process without them forgetting to show you something that's integral to their workflow because they take it for granted?

Do you charge for it or do you take it as a cost to score a contract?

Do you have a questionnaire with all the potential features/reports they might need but will definitely forget to tell you themselves?

Have you had any bad experiences that you succesfully learned from and adapted to? How?

Thanks!