One aspect of interpersonal communication that is revealed in Fireproof suggests that true communication between two people cannot take place unless there is a full understanding of the other. Prior to Caleb's embrace of the "Love Dare" program and the spiritual tenets of Christianity, there is little communication between he and Catherine. Both of them are filled with resent towards the other, and their communication is filtered through this prism. When Caleb embraces the true tenets of Christianity and channeling his identity into something higher and more substantive than his own notion of self, true communication happens. While Catherine might doubt it, when she becomes ill and is forced to confront the "new" husband she has because of the "Love Dare" program and the redemptive power of Christianity, true communication is only possible at this point.
Another aspect of interpersonal communication that is show in the film would be the relationship that one has with something larger than themselves. Caleb is only able to truly communicate when he establishes communication with the divine. Prior to this, Caleb had used substitution for communication. Internet porn and the desire for the boat were other directed ends that substituted for real communication. When Caleb begins communion with the divine, he removes these elements in favor of real communication and the interpersonal communication that results is rooted from his own communication with the divine. It is only when this true form of communication happens that stronger interpersonal communication is realized.