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The usual methods in Maya for starting IPR also work.  The only difference is that the button in the RenderMan shelf always switches to RenderMan before starting a render.

You may also make use of the Interactive Refinement control in the Render Settings to let RenderMan decide how often to update pixels, providing you with a faster preview that updates or "refines" over the next few iterations, filling in pixels.

The IPR button offers a menu with some convenient options for starting up with a particular camera or resolution.

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