This paper is on the foundations of a recent approach to the design of massively parallel and interactive programming languages using rv-systems (interactive systems with registers and voices) and Agapia programming. It includes a few theoretical results on FISs (finite interactive systems), the underlying mechanism used for specifying control and interaction in these systems. First, we give a proof for the undecidability of the emptiness problem for FISs by reduction to the Post Correspondence Problem. Next, we use the proof to get other undecidability results, e.g., for the accessibility of a transition in a FIS, or for the finiteness of the language recognized by a FIS. Finally, we present a simple proof of the equivalence between FISs and tile systems, making explicit that they precisely capture recognizable two-dimensional languages.