1. A method allowing computer-generated speech to be rendered with a multi-voice font having different speaker characteristics and/or prosody than source voice fonts used to generate the multi-voice font, the method comprising the acts of: loading the source voice fonts; assigning weights to characteristics of each source voice font relating speaker characteristics or prosody; obtaining text to be rendered as the computer-generated speech; predicting characteristic values for the text using each source voice font; and merging the predicted characteristic values with the corresponding weights to produce interpolated characteristic values ready to be used for rendering the text as computer-generated speech.