Different game runners operate differently. In pre-prepared bought "modules" (adventures) there are usually full character descriptions and gaming stats for every villain and cast member, maps of areas, and descriptions of contents of rooms including trasures and clues. There are tables of random events and encounters so the DM can be as surprised as the players how a journey might work out; and so on.

Some DMs like to prepare in equally meticulous detail, with cards or sheets for all the main characters that the players will encounter, detailed plot diagrams etc.

The idea is to be prepared enough to respond to whatever the players decide their characters do. In a computer game there are always a limited number of options. Do you charge in through the front door or sneak in through the back door? In a live game the players might choose to go in through the roof, or set fire to the haystack next door as a diversion, or go somewhere else entirely. The DM has to be ready with what happens then.

I've sometimes written out roleplaying plots far more complex and complete than anything I've ever plotted for a text story; other times I just wing it and see what develops. In this scenario's case I had a broad idea of what the overall storyline was about - it'll be evident by the end of chapter four - a half-page list of things in Josiah's cottage and where they were now, and a list of thirteen words which cover the main plot points.

You'd probably enjoy roleplaying. It tends to appeal to problem-solvers and tactical thinkers.