The aim of the work described in this paper is to extend the EPFL dialogue platform with multimodal capabilities. Based on our experience with the EPFL Rapid Dialogue Prototyping Methodology (RDPM), we formulate precise design principles that provide the necessary frame to use the RDPM to rapidly create an efficient multimodal interface for a given application. We analyze the consequences of the proposed design principles on the generic GUI and architecture required for the system.