Designers have limited tools to prototype AR experiences rapidly. Can lightweight, immediate tools let designers prototype dynamic AR interactions while capturing the nuances of a 3D experience? We interviewed three AR experts and identified several recurring issues in AR design: creating and positioning 3D assets, handling the changing user position, and orchestrating multiple animations. We introduce Project Pronto, a tablet-based video prototyping system that combines 2D video with 3D manipulation. Pronto supports four intertwined activities: capturing 3D spatial information alongside a video scenario, positioning and sketching 2D drawings in a 3D world, and enacting animations with physical interactions. An observational study with professional designers shows that participants can use Pronto to prototype diverse AR experiences. All participants performed two tasks: replicating a sample non-trivial AR experience and prototyping their open-ended designs. All participants completed the replication task and found Pronto easy to use. Most participants found that Pronto encourages more exploration of designs than their current practices.