Computationally testable models in linguistics focus on declaring data structures and providing exemplar derivations. This paper outlines a comprehensive model of speech production which goes beyond derivations to show how actual instances of utterances can be formally characterised. Utterances contain a wealth of detail beyond the underlying utterance plan: some of this is a function of the mechanism itself (e.g. coarticulation) and some is the result of carefully supervised control. We develop the notion of managed or supervised speech production to enable the inclusion of EXPRESSIVE content in speech. Building on earlier work the Cognitive Phonetics Agent bridges the gap between the physical and cognitive processes in phonetics by controlling the way phonologically determined utterance plans are phonetically rendered in detail. The model is illustrated using different types of data structure which occur in speech, concentrating in particular on an XML characterisation of appropriate structures. We trace a simple utterance through from its phonological plan to a detailed intrinsic allophonic representation to show how stages in the model work.