A key feature of the Rational Unified Process is its approach to incremental integration throughout the lifecycle.
During the elaboration phase, you build an evolutionary, structural prototype that becomes the final system during the construction phase.
In parallel, you can build a few throwaway behavioral prototypes to explore issues such as the user interface.
Round-trip engineering is a technique, supported by a tool such as Rose or XDE, that closely ties together the design and the implementation of a software development effort.