During early phases of the development of an interactive system, future system properties are identified (through interaction with end-users, e.g., in the brainstorming and prototyping phases of the development process, or by requirements provided by other stakeholders) imposing requirements on the final system. Some of these properties rely on informal aspects of the system (e.g. satisfaction of users) and can be checked by questionnaires, while others require the use of verification techniques over formal models (e.g. reinitializability) or validation techniques over the application (e.g. requirements fit). Such properties may be specific to the application under development or generic to a class of applications, but in all cases verification and validation tools are usually required for checking them. The usability of these tools has a significant impact on the validation and verification (V&V) phases which usually remain perceived as very resource consuming. This chapter proposes a user–centred view on the use of formal methods, especially on the use of formal modelling tools during the development process of an interactive system. We propose to apply Norman’s action theory to the engineer tasks of formal modelling in order to analyse how modelling tools impact the following engineer activities: model editing, model verification and model validation. The CIRCUS Integrated Development Environment serves as an illustrative example to exemplify how engineer tasks may be impacted by modelling tools features.