An execution view is an important asset for developing large and complex systems. An execution view helps practitioners to describe, analyze, and communicate what a software system does at runtime and how it does it. In this paper, we present an approach to define execution viewpoints for an existing large and complex software-intensive system. This definition approach enables the customization and extension of a set of predefined viewpoints to address the requirements of a specific development organization. The application of this approach has helped us to identify, a set of execution viewpoints that we are currently using to construct execution views of an MRI system, a large software-intensive system in the healthcare domain