The present study aims to capture linguistic variation in subject distribution of two typologically similar languages, Greek and Chilean Spanish, considering adult monolingual and bilingual speakers of Greek as a heritage/minority language in Chile. The focus is on null and overt third-person subjects in topic-continuity and topic-shift contexts. Such structures involve the interface between syntax and discourse/pragmatics, a vulnerable domain in bilingualism. Previous research has shown overextension of the scope of the overt subject pronoun in contexts where null subjects are discursively expected (e.g. Tsimpli, Sorace, Heycock & Filiaci 2004). The Interface Hypothesis (IH) (Sorace 2011) was formulated to account for such findings, which obtain even in pairs of two null subject languages (Sorace, Serratrice, Filiaci & Baldo 2009). The key question as to the language-contact effects on subject distribution in pairs of two null subject languages requires further exploration while the combination of Greek and Spanish has been so far understudied. The IH is evaluated with new empirical data from a bilingual situation not studied before. Data from oral narratives and aural pronominal anaphora resolution were elicited from monolinguals and three types of bilinguals, namely first-generation immigrants, heritage speakers and L2 speakers of Greek residing in Chile. The monolingual data revealed differences in the use and interpretation of overt subject pronouns between Greek and Chilean Spanish. The crosslinguistic difference lies in the strong deictic properties of the Greek pronoun compared to its Spanish counterpart; hence differences obtain because of the relative strength of the two pronominal forms. No overextension of the scope of overt pronouns was found in bilinguals, against predictions stemming from the Interface Hypothesis. This may relate to the typological similarity between Greek and Spanish as well as to the nature of the Greek pronoun, which makes its use relatively categorical. Such findings lend support to the Representational account (Tsimpli et al. 2004). On the contrary, null subjects gave rise to optionality presumably due to their complexity, which demands higher degrees of computational efficiency. The Vulnerability Hypothesis (Prada Pérez 2018) may also account for the findings.