In environments where time is not normally constrained, and more often than not in the context of large enterprise level projects, if you are asked to join the project for a couple of weeks then before agreeing, be extremely clear what is being asked of you. What you are actually being asked to do is to deliver the interface.
The problem here is that some questions may have already been answered but the broader context has not yet been established. A solution can obviously be designed within a two week period, but to agree to this without establishing the scope means that you will only create more work and pain for yourself later.
If so-and-so has already started and you are being invited to just finish them off, why hasn’t so-and-so been able to do that? Design is an iterative process and it usually signifies that something immovable has been identified and needs solving or something isn’t working out.
Some, not all, Project Managers do not get this and are only interested in limiting or constraining time to achieve something tangible. If you are being asked to deliver the UX it means that fundamental questions have not yet been raised or even answered.
The difference between the interface and the UX in this instance assumes that key questions of the experience have already been answered. Invariably these haven’t! Otherwise the project would be able to start creating designs.
Once you create something, it naturally invites feedback. So unless things like navigation metaphors, user journeys and outcomes are documented, you are being asked to do UX not design. UX takes time to comprehend and make recommendations; the UI is just doing.
Frazer Cooper 