In their influential book on SUIs, Balentine and Morgan stated that the main enemy of the spoken user interface is time. The basis for this assertion is that speech has a temporary existence and listeners must remember what they have heard. If prompts in a speech application are too short, however, they can be subject to multiple interpretations. A clear design objective, therefore, is to avoid making users hear more (or less) than they need to hear or to say more (or less) than they need to say.
It is also important to strive to make every interaction move the user forward (or at least create the illusion of moving forward). This is easier said than done because dialogs need careful crafting and usability evaluation. To work toward these objectives, use prompts that are succinct and sincere (modeled after the prompts provided by expert call center agents), provide self-revealing contextual help messages, and use a professional voice talent for recorded prompts.