1. A method comprising: identifying, via a processor, a speech synthesis context; determining, based on a local cache of text-to-speech units for a text-to-speech voice and based on the speech synthesis context, additional text-to-speech units which are not in the local cache; requesting from a server the additional text-to-speech units; receiving the additional text-to-speech units from the server; and synthesizing speech using the text-to-speech units and the additional text-to-speech units.
5. The method of claim 1 , further comprising: determining parameters relating to speech synthesis; and determining, based on the parameters, how many additional text-to-speech units to request.