Formal systems modelling offers a rigorous system-level analysis resulting in a precise and reliable specification. However, some issues remain: Modellers need to understand the requirements in order to formulate the models, formal verification may focus on safety properties rather than temporal behaviour, domain experts need to validate the final models to ensure they fit the needs of stakeholders. In this paper we discuss how the principles of Behaviour-Driven Development (BDD) can be applied to formal systems modelling and validation. We propose a process where manually authored scenarios are used initially to support the requirements and help the modeller. The same scenarios are used to verify behavioural properties of the model. The model is then mutated to automatically generate scenarios that have a more complete coverage than the manual ones. These automatically generated scenarios are used to animate the model in a final acceptance stage. For this acceptance stage, it is important that a domain expert decides whether or not the behaviour is useful.