||We introduce Side Views, a user interface mechanism that provides on demand, persistent and dynaic previews of commands. Side Views are designed to explicitly support the practices and needs of expert users engaged in open ended tasks. In this paper, we summarize results from field studies of expert users that motivated this work, then discuss the design of Side Views in detail. We show how Side Views' design of Side Views in detail. We show how Side Views' design affords their use as tools for clarifying, comparing, and constrasting commands; generating alternative visualizations, experimenting without modifying the original data, and as tools that support the serendipitous discovery of viable alternatives.