The interaction designer
UX != UI You need a experienced interaction designer to deliver good user experience, contrary to popular belief that is not a programmer. InFor all of you programmers who think they can do UX (that includes me) let me say this. Getting good at interaction design requires at least as much time as getting good at programming. How much time have you spent doing pure interaction design?
In the initial phase it's his/her responsibilitythe interaction designers responsibility to:
- Extracting the actual goals of the solution from the product owner
- Define the personas that will be using the solution.
- Write scenarios that are stories on how the solution will be used.
- Compile a design document that leaves little room for ambiguity from an UX standpoint
During the project it's the interaction designers task to make sure that those guidelines are followed and address any additional issues that will arise (and they will).
Many programmers will take afront to this approach I'm sure since everyone feel that they're the exception that can design "wonderful" interfaces, you're probably not. On the other hand a good interaction designer - programmer relation is often very nice for the programmer as well as they don't have to fight against a "stupid specification". Unfortunately good interaction designers are hard to find in my experience but they are out there.
As always I deeply recommend Alan Coopers books on the subject ("About Face" and "The Inmates are running the asylum")