Three interactive tasks, increasing in the complexity of resource-directing reasoning demands on speaker/storyteller attribution of, and linguistic reference to, the thoughts and intentions of characters in narrative stimuli were performed by Japanese L1 speakers of English. Largely consistent with the claims of the Cognition Hypothesis, results of the present study show; (i) task complexity led to more complex speech assessed using specific measures motivated by the conceptual/linguistic demands of the tasks but did not, however, affect accuracy, fluency and complexity assessed using general measures; (ii) tasks requiring complex reasoning about characters' intentional states led to significantly more interaction, and uptake of premodified input than simpler versions; and (iii) output processing anxiety showed a linearly progressive, negative relationship to use of complex speech as tasks increased in complexity. The role of specific versus general measures of production is discussed, as is the importance of examining interactions of production, interaction and uptake with measures of individual differences when researching the influence of L2 task demands on learning and performance. (Contains 7 tables and 1 figure.)