Active alignment describes a morphological alignment pattern where the lone argument of an intransitive verb is marked sometimes like the subject of a transitive verb, and sometimes like the object. Many generative accounts of active alignment hold that this morphological distinction is rooted in the syntactic distinction between external arguments, merged as the specifier of a functional head Voice (or v), and internal arguments, merged as an argument of the lexical verb. However, on the basis of novel fieldwork with Choctaw, a language with an active agreement system, I show that an argument’s morphological marking must be dissociated from its syntactic position: the marking that is characteristic of canonical external arguments is, exceptionally, found with certain internal arguments too. Nevertheless, I show that these internal arguments receive their exceptional marking only if they can form an uninterrupted syntactic (i.e. Agree/case-assignment) relation with the Voice head.

The implications of these findings are twofold. Firstly, active alignment is argued to be a consequence of Voice forming a syntactic relation with some arguments and not with others, rather than a direct consequence of the differing syntactic positions of internal vs. external arguments. This provides a new way of understanding lexical and configurational exceptions to the dominant alignment pattern of a language. Secondly, by studying the particular circumstances under which internal arguments receive exceptional marking, I argue that the agreement/case-assignment properties of a single Voice head can vary contextually according to the syntactic material in its immediate neighborhood, including the lexical root and other functional heads. This brings the agreement/case-assignment properties of functional heads in line with how we often think about their morphological properties: that is, they can have default and contextually-conditioned variants.