Automaticity
To achieve any degree of fluency, some degree of automaticity is necessary.
At the level of formulation, automaticity is partly achieved through the use of prefabricated chunks.
At the discourse level, a degree of automaticity is possible due to being familiar with a particular genre, topic and interlocutors. embarking on a completely untried speech genre, on an unfamiliar topic, with unknown interlocutors, is likely to make one 'tonguetied'.