This dissertation is a theoretical investigation of the thesis of Minimalist Interfaces, namely, that syntax-external linguistic interfaces that interact with the core syntactic computation and language-independent sound and concept systems play a more critical role in manipulating syntactic objects to make them legible to those systems than is currently assumed in the recent minimalist inquiry. The core theme of this thesis lies in the idea that syntax is not entirely crash-proof but could make a variety of derivational mistakes; phonological and semantic linguistic interfaces conduct a handful of independent domain-specific operations to attempt to legitimize illicit syntactic objects, if any, for the purposes of legibility at the language-external sound and concept systems. Evidence is provided that the syntax-external components use whatever resources they can to repair certain "imperfections" created by syntax but only within the range of options made available by the universal principles of syntax in tandem with the language-specific parameter values. This dissertation explores some of the ramifications and empirical consequences of this thesis based on the comprehensive description of a sizable portion of the grammar of Indonesian and Javanese collected by my fieldwork with three native Indonesian and Javanese consultants. Phenomena discussed here include the distribution of active voice morphology, P-stranding under sluicing, the denotation and morphosyntax of bare nominals, wh-in-situ questions, and reduplication asymmetries between nominal and verbal derivational affixes. These diverse ranges of phenomena in the two languages are analyzed in depth to provide converging evidence that the thesis of minimalist interface as defined above yields a deep understanding of the way the syntax interacts with the language-dependent interfaces responsible for phonological and semantic interpretation. The investigation conducted here, necessitates serious reconsideration of the commonly held view of linguistic interfaces as passive, merely ornamental components of natural language grammar ruled by the universal law of syntax.