Understanding the role of input in bilingual phonological acquisition is revealing for deciphering the workings of language acquisition processes. Input and usage distributional frequencies guide and differentiate speech sound acquisition patterns cross-linguistically. Such processes are operant in first- and second-language acquisition. There is an under-representation of investigations on how context-specific input in bilingualism influences the early acquisition of rhotics in child developmental speech longitudinally. This study addresses the gap by tracing a Greek/English bilingual girl’s rhotic development between ages 2;7 and 3;11, utilizing naturalistic data during daily interactions with an adult interlocutor. The study reports and schematically illustrates the child’s bilingual usage frequencies, informing language choice in her production variables, which demonstrate, quantitatively and qualitatively, the effects of context-specific input on rhotic accuracy levels and substitution patterns in both languages. Specifically, distributional frequencies in the input govern the child’s phonemic and phonetic tendencies in the languages. Findings are compared with previous reports in the literature and enhance language acquisition theory, revealing the pivotal role of input in the dynamic interplay with developmental universals, language-specific tendencies, transfer, and individual variation.