Anxiety is a major obstacle to learning a foreign language @1991horwitz_foreign. It has effects on several aspects of language use and language learning. Anxiety can affect how students perceive the language, how they attempt to interact in the language, the amount of errors they produce, and their study habits.
Regardless of the model of language acquisition one follows, anxiety will be a limiting factor. If one takes a view that impromptu conversation drives language acquisition (@1972savignon), anxiety poses problems in that students are limited in their production of language as well as their perception of it. If one takes the view that deriving meaning through input (@1980krashen) drives acquisition, we are still concerned with anxiety because of its ability to change how students perceive the language.