Any adventures I write are usually pretty free form. I give a direction, and some unchangeable events, but leave a lot of wiggle room. Players almost never respond how one might think.
My preparations for a game are usually pretty intense. I like to immerse people as much as I can. I construct a soundtrack, item cards, numerous NPC cards, and custom miniatures. I also usually have multiple story arcs within the frame of a larger story and often ask the players to write a background so I can include a storyline that's centered around that specific character. A friend of mine tried to pick up as GM a Star Wars game I had to abandon and wasn't able to keep track of what I was doing. We had six players with four major intersecting story lines and about two dozen NPCs from all over the galaxy, each with their own background, motivations, and opinions of the characters. I also had a destination for some of the NPCs; some could be convinced to join the Rebellion, for example.
As stated, I like to fully immerse my players. I like the roleplay, not dungeon crawl. Most games I've played in (not as a GM) are usually some variation of playing Diablo. Not that dungeon crawls can't be fun. Murdering hordes of baddies has its place, even in my games. But I like to have my players feel like they've actually been on a journey and have had some sort revelatory experience.
Maybe it's just the natural progression of my love for epic stories.