A supernova first becomes visible as it breaches the surface of its host star, and transitions from a stellar explosion into an expanding remnant. Launched in this event are a flash of photons and a spray of very fast ejecta that interact with circumstellar matter (winds, disks, companions) to produce transients and afterglows. Highlighting a few notable examples, I will discuss how these observables encode information about the star's structure, its immediate surroundings, and the nature of its explosion. Optically thick winds and relativistic shocks create distinct classes of transients. I will argue that aspherical explosions can depart dramatically from their spherical analogs, producing a novel type of outburst. This range of possibilities continues to present a challenge to theorists and an opportunity for observers.