Knowing how to prepare for a session is an important skill for Dungeon Master to have. However, I've often heard about the dangers of overpreparing for a session and how it can lead to railroading. Spending too much time preparing is also an inefficient way to spend your time. There is a lot to be said on this topic and I hope sharing my current thoughts on the matter will help someone.
Not Every Path
I assume during this piece that we are not talking about someone who is trying to predict every path an adventure might take and run it like an in person computer role-playing game. It's a lot of work and often gets invalidated anyway because a person might thing of a solution or option you never considered.
Who Am I?
People can be very different and this also has an effect on overpreparing. I've seen people who would just run a game of D&D on the spot because someone asked them to. Of course, in this case it wasn't a pre-published adventure. They had enough games under their belts that they could just run a concept that they've always wanted to run but haven't gotten the chance to yet (it was also theatre of mind with no rulebooks and with new players). Not everyone can do this (in fact I'd be very careful with even trying). New Dungeon Masters who don't have the experience will probably need considerably more time to get to the same level of preparation as an experienced one and may even need to quite commonly reference the rules, further slowing things down. Not all experienced Dungeon Masters will prepare equally, both in terms of time and quantity of things they prepare. Strengths and weaknesses need to be factored in. For these and other reasons, quantifying “overprepared” is not easy. As you get a chance to run games, you learn more about yourself and are able to figure out what you need.
All of our time is limited and some of us have more time than others. At a certain point, preparing more won't improve the session. Before this point there will also be a point where preparing more just isn't worth the investment even if there is an extremely tiny benefit. There are other things to do, maybe even some things that need to be taken care of for future sessions.
Tabletop role-playing games, regardless of what system you use or who you play with, are supposed to be fun. Don't treat it like a test where you need to cram as much as you can (also, if you are like me this won't work anyway and you are better off getting a good night's sleep). Regardless of how long you spend to prepare, there will be some guy (probably a bard or rogue) who thinks of something else.
What Stage Are
Different parts of a campaign or even an adventure will need more or less time. I tend to take more time at the start of the campaign than many other parts. This is because I want to have a rough idea of what my themes are, what characters are in my world, to have some fallback characters ready in case my players try to talk to a minor character, and to develop the setting the campaign takes place in. Even if you set your campaign in an already established setting, you will need to fit your own story into the world, assuming you aren't running a published adventure. If you are, you will probably also spend a considerable amount of time at the start reading the adventure and making notes.
Running a standalone adventure is also a skill. Different people may allocate their preparation time differently depending on how comfortable they are with certain parts. I tend to take more time with encounters I want to be memorable. That way I know what I'm doing when I jump in. I'll also spend more time on maps I find particularly troublesome or confusing. You also can't rule out needing to make your own hotfix or modification to some portion of the adventure. The longer adventures are closer to a campaign in terms of length but pure creation and interpreting a published adventure are different beasts.
The Root of the Problem
I don't think that preparing for too long needs to lead to railroading. I think the real root of the problem is getting too attached to the plan you made. If you spend a long time preparing a session, you might get attached to your plan and not want to stray. If you did overprepare, there is a chance that you wasted some of your time. That's fine. I've seen quite a few people in my time that had no issue abandoning their plans if things just didn't go that way. Just because you don't use what you've prepared this session doesn't mean you never will. Write it down and file it away somewhere. Even if you don't use it in the exact same form, you can still find some use for it later and as a result it won't go to waste. There will be some prep-work that will be closely tied to the adventure and hard to reuse. Keep this as small as you can but you will always need some. If you go too far, you'll have probably wasted some time if things go off the path (unless you can draw something from the experience, get more ideas, or reuse it cleverly later) but the session can still go well if you just roll with it. It's sometimes just part of the process.
Of course, this all assumes you'll be running more than one session. You might still have this problem if you are the Dungeon Master for a one off adventure and then plan to return being a player forever after. To this point I say: beware. You may find the role of Dungeon Master will find you again. And when it does, you might find something of use in what you didn't use before. Sometimes your rejected ideas also make for good inspiration for new characters that you want to play.
What Should I Prepare?
I prepare the following at the very least:
- Rough story line including what will happen, if anything, if players don't take the bait (a couple of alternate outcomes are probably fine, but don't dig down too deep)
- Combat encounters I want to be memorable or are more complicated (these two tend to go hand in hand)
- Look over the big characters and understand them (big ones, not an NPC that will show up for 5 minutes)
- Have some side characters you can throw in just in case (a list with a few names and a printout of a stat block or two from the basic rules, possibly with a couple sentences of notes, is usually enough)
- Have the maps in some sort of easy to reference manner (photocopy, printout, bookmarks, whatever)
- Find some tiles, maps, and miniatures for the session (I tend to use them so I need them ready to go)
- If it's been a while or I'm running multiple systems, a quick skim of the rules
You may need to prepare more or less. Depending on how into miniatures you are, you could spend more time preparing a set piece.