Software product line engineering is often a more laborious process than anticipated beforehand, not in the least due to a growing demand for product features and an ever increasing complexity of the dependencies between functional components. One of the main ideas in software product line engineering is to delay variant binding, i.e., to delay the composition of particular product features to a later moment in the development or deployment process. Delaying variant binding affects testability, e.g., a full integration test is not possible before all the appropriate product variants have been bound. This paper suggests a Variability and Testability Interaction Model (VTIM) to better anticipate the software product line testing process. VTIM is applied in a case study in expressing the relationship between variability and testability for several variation points in a large-scale software product line of magnetic resonance imaging scanners developed by Philips Medical Systems. The case study illustrates how VTIM can be used as an analysis tool in everyday software engineering practice.