No different in landscaping - just more dollars involved.
I few years ago I had a guy and his wife have me out to their house - they had built retaining walls from the mini-block homeowners are so fond of. They also had some of those 18"x18"x1.5" patio block made to look like a paver pattern. They wanted us to rebuild the walls in a different manner, and wanted suggestions on the patio block. I talked them out of the patio block, and they wanted to have the store they bought them from pick them up and swap them for some other choice. And they wanted to do it all T&M. I told them I'd stop out at that store to look at the selection, and work that into a design. I also told them 2+ weeks for the design process.
36 hours later I get an angry call - "We had the old block picked up - it sure would have been nice to know what materials we were going to swap it with before they picked up the old. Get back to us as soon as you can so we can talk about this problem."
So I did - I told them "I don't think we'll be able to live up to your expectations, so it will be in your best interest to find a contractor who can."
We've lots of stuff like that, but that one stands out in my memory at the moment.