This is the ongoing making-it-real of the experience strategy and vision. The UX Designer engages throughout the entire process, and makes sure that the user experience is never being compromised. If circumstances arise that force changes, the UX Designer leads the discussion to make sure that the new solution still adheres to the overall strategy. The UX Designer must sweat every detail, and, yes, occasionally be a jerk about it.
Coordination between the stakeholder, developer and myself (as UX and Art lead) were crucial in making sure all needs were served, and compromising where we could due to time constraints.