This paper presents Whizz, a system for building animated interactive applications. We describe its musical metaphor, and its underlying model based on streams and events. We analyze the notion of animation, and show how the Whizz model can be applied to the whole dynamic behaviour of an interface, including animation, user input, and communication with the application. Such an integration leads to a homogeneous definition of that behaviour, and to interactive tools to define it. It also opens doors on new types of interaction that mix users' actions with animation.