The current complexity of systems requires a full understanding of stakeholder needs in system design. Later, these needs will become requirements of the system: functional requirements are quite easy to describe for the customer, but finding the non-functional requirements is an actual challenge; however, they are essential to design the system. The objective of this research work is to provide support when defining non-functional requirements. The proposal consists in a requirement classification and a questionnaire that progressively guides the elicitation of stakeholder needs. This method is applied to three case studies, demonstrating its interest and indicating opportunities for improvement.