Cascading failures are often results of a single (initiating) event, followed by inappropriate control action/s. Adequate assessment of power system responses and vulnerabilities can only be achieved with proper modeling and representation of critical elements and crucial control actions in power system analysis tools and procedures. Despite the well-understood importance of representing critical system elements in power system analysis to improve situational awareness and to hedge against Mother Nature and malicious attacks, many research efforts in these directions often employ steady-state analysis. The purpose of this paper is to demonstrate that methods based purely on steady-state analysis may fail to identify the most critical system vulnerabilities. This research also emphasizes that the most essential components of a power system should be investigated and modeled in all transient based studies in order to have a more realistic estimation of system behavior. This paper presents a systematic method to determine critical protective relays that need to be modeled in transient stability studies.