Customizable software systems consist of a large number of different, critical, non-critical and interdependent configurations. Reliability and performance of configurable system depend on successful completion of communication or interactions among its configurations. Most of the time users of configurable systems very often use critical configurations than non-critical configurations. Failure of critical configurations will have severe impact on system reliability and performance. We can overcome this problem by identifying critical configurations that play a vital role, then provide a suitable fault tolerant candidate to each critical configuration. In this article we have proposed an algorithm that identifies optimal fault tolerant candidate for every critical configuration of a software system. We have also proposed two schemes to classify configurations into critical and non-critical configurations based on: 1) Frequency of configuration interactions (IFrFT), 2) Characteristics and frequency of interactions (ChIFrFT). These schemes have played very important role in achieving reliability and fault tolerance of a software system in a cost effective manner. The percentage of successful interactions of IFrFT and ChIFrFT are 25 and 40% higher than that of the NoFT scheme. In NoFT scheme none of the configurations are supported by fault tolerance candidates. Performance of IFrFT, ChIFrFT, and NoFT schemes are tested using a file structure system.