Marcus Blankenship and I wrote an article, Stay Agile with Discovery, to discuss how to help your clients see the benefits of working in an agile or more agile way.
We have seen too many clients want “agile” and not want all the responsibilities that being a Product Owner or customer involves. If your client asks you to be agile and then demands you estimate “everything” and provide a fixed cost, fixed scope “agile” contract, you don't have to say, “NO.”
You can say, let's try a discovery project so we (as the provider) can explore what it would take to do “everything.” As we finish this first discovery project, where we will provide working product, you can provide us feedback. Based on that feedback, we might do another discovery project. In fact, you can work in month-long (or two-week long) discovery projects all the way through. Your client can ask for changes that you incorporate into the next discovery.
That's just one way to help people learn about collaboration and resilience over contracts and guarantees.