Here's a video from BMW which delves into the evolution of the current Z4's design.
We get to meet the woman responsible for the Z4's exterior design, Juliane Blasi, and she takes us through her thought processes on the development of the model.
The video shows how sketches turn into clay models and then get produced into prototypes.
One of the issues with the Z4 Roadster was in making that roof work quickly and effectively while looking good at the same time.
A cool job but difficult - and one that comes with a large amount of pressure to meet engineering requirements while at the same time creating something that is pleasing to the eye of the consumer.