(Note: I feel that this question is sufficiently different from this one to justify a new post, as my question is about specific information informing Wizard spell preparation)
(Additional note: These same issues may be similarly relevant for other preparation casters. I chose to focus on Wizards because of my personal experiences as a player, my current needs as a DM, and the flexibility in access to spells that is unique to Wizards.
Please feel free to address this question for any preparation caster; Wizards simply have more opportunity to make "bad" choices in preparing spells due to their large spell list and greater number of spells known, while also having one of their key features be their versatility in what spells they can bring to a challenge)
One of the major class-balancing elements of the Wizard class is that they are only able to prepare a certain number of spells at a time. Among all magic using classes, wizards have access to the largest number of spells that they can learn, which makes them extremely versatile, but that versatility is tempered because they can only prepare a subset of those spells at any given time.
This can be very exciting-- it adds a degree of planning, creativity, coordination, and tension to playing a wizard. It is also valuable in that it helps prevent the wizard from overshadowing other spellcasters with less expansive spell lists.
However, it can also be frustrating, particularly at lower levels when fewer spells can be prepared at once. It's difficult to choose which spells to prepare if you don't have any idea what challenges you'll be facing, and even if you do have some information about that there may not be enough time to change which spells are prepared beforehand. It can be annoying to be completely mis-prepared for challenges.
Sometimes that's appropriate: surprises do happen, narrow windows of opportunity may appear, and being maximally appropriately prepared for every encounter is not a realistic goal. Often there will be a middle ground, such as knowing you'll be exploring an area rumored to be filled with undead enemies and so it might be a good idea to have spells that are useful against the undead.
But it's not much fun to only rarely be able to use spells that you've learned specifically to deal with situations in which you find yourself because you didn't (whether or not you couldn't) know anything about what you're walking into. Using magic to solve problems is largely what the Wizard class is about.
This has frustrated me as a player, and I'm having trouble balancing those concerns in a game I'm running now. If my approach to the issue is fundamentally that:
- Surprising encounters can happen, particularly if my players drive those events
- Events which are known in advance may not offer enough time to swap out prepared spells before dealing with them
- Gathering advance knowledge about upcoming challenges won't always be equally possible or reliable
- Obtained knowledge about future challenges shouldn't be a how-to guide on min-maxing those challenges
- I don't want to do away with the preparation mechanic
- I don't want to elide the issue by providing abundant spell scrolls or similar items
My experiences playing a wizard have been plagued by this issue, often leading me to wish I'd just chosen a different class. I'd like to spare my players that irritation if I can.
How can I telegraph enough information to a player with a preparation caster character (especially at lower levels) that they can avoid being totally mis-prepared, and how often should I do so?