In recent years, the psychological aspects of safety have begun to attract attention in human-robot interaction (HRI). In this paper, we present an interview guide that can help to investigate perceived safety by revealing the underlying reasons for feeling unsafe during HRI. We used six factors that are identified to be closely related to perceived safety in the literature. These factors are context of robot use, comfort, experience and familiarity with robots, trust, sense of control over the interaction, transparent and predictable robot actions. The interview guide includes detailed questions to cover these concepts. The paper also shows an example application of these questions with thirteen participants and the social robot Pepper. The interaction scenario was playing a quiz game which included controlled modifications at specific moments to stimulate lower levels of perceived safety. The analysis of interviews showed that the questions reveal critical aspects influencing perceived safety, aligning with findings from other HRI studies. This interview guide can help researchers identify the influencing factors of perceived safety in HRI for various types of robots and use-cases and facilitate practical implementations of the identified factors.