This paper contains three studies of factors which affect the choice of words in simple speech-based interactions. It is shown that choice of words is affected by the level of constraint imposed on users, such that variability is much higher when no constraint is applied than when some form of constraint is used and that variability can be reduced by employing different forms of feedback. In particular, the design of visual and auditory feedback has a bearing on users' choice of words. However, it is proposed that these results do not necessarily lead to people copying the computer, but arise from users developing appropriate communication protocols in their transactions. The paper concludes that choice of words will be subject to a number of factors, that some of these factors can be modified through system design but that 'out-task' vocabulary or inappropriate use of commands can still present problems. Until we have a better understanding of the linguistics of speech-based interaction with machines, these problems will remain intractable.