We present Astral, a prototyping tool for mobile and Internet of Things interactive behaviours that streams selected desktop display contents onto mobile devices (smartphones and smartwatches) and remaps mobile sensor data into desktop input events (i.e., keyboard and mouse events). Interactive devices such as mobile phones, watches, and smart objects, offer new opportunities for interaction design– yet prototyping their interactive behaviour remains an implementation challenge. Additionally, current tools often focus on systems responding after an action takes place as opposed to while the action takes place. With Astral, designers can rapidly author interactive prototypes live on mobile devices through familiar desktop applications. Designers can also customize input mappings using easing functions to author, fine-tune and assess rich outputs. We demonstrate the expressiveness of Astral through a set of prototyping scenarios with novel and replicated examples from past literature which reflect how the system might support and empower designers throughout the design process.