1. Features of the context:
   1. the listener - it is easier for the speaker if the listener is one of his peers or 'junior' to him. It is easier for him to talk to one listener than to many.
   2. the situation - it is easier for the speaker if he is speaking in a familiar, private environment.
      
      
2. State of knowledge of the listener:
   1. the language - it is helpful for the speaker if the listener knows as much of the target language as the speaker does.
   2. the information - it is helpful for the speaker if he has information which the listener does not have but which the listener, for some reason, needs. This puts the speaker firmly in control of the information and motivates him to communicate that information.
      
      
3. Type of task:
   1. status of knowledge - it is helpful for the speaker if the information he has control of is of a familiar sort so that he understands it thoroughly. It is helpful if he is familiar with the foreign-language vocabulary which is essential to the completion of the task.
   2. structure of the task - it is helpful to the speaker if the information in the task provides its own structure so that the language is externally supported by the requirements of the task. Thus it is easier for any speaker to give an account of a series of events than it is to provide an argument for why those events occurred in that order.

(From Brown and Yule, 1983b: 34)