There are two important issues here - getting players to
try out each character, and getting players to
frequently use each character. Not all games need to encourage both. And these issues apply not only to really large casts, but to
any game with a cast of playable characters that is larger than the active battle party!
Try Out Each Character
Most games want to encourage players to at least try out each character, and that's easier said than done because players will often find and stick with a party that works well, rather than deal with the risk and extra learning of incorporating new party members into the mix. This is especially true in Action Battle Systems where characters might control and play very differently from each other (which is super-nifty, but presents a bigger barrier to learning new characters' kits).
The most straightforward way to handle this - and usually the best in my opinion - is to simply
force the character into the player's party for a little while after they join your party. This is usually when the character is in the spotlight story-wise, so it makes sense that they'd be part of the action. For one dungeon or so after the character joins, lock the character into the party (don't let the player remove them). The player will naturally learn and understand the character's basic battle style, and if they enjoy it, they will start using it from then on (and if not they'll remove the character as soon as the choice opens up). To compensate for the player's initial ignorance of the character's battle style, try to make dungeons and bosses with forced party members a little easier than normal.
If your game (or story) doesn't suit well to forcing characters into the player's party, then consider having different "accomplishments" that are specific to each character (Harold might have ones like "Finish an enemy with Fireball" and "Dodge a lethal blow" - Harold must do these things to get credit, not another party member). Accomplishing these might give you rare in-game items, or Trophies/Gamerscore. This won't encourage everyone to try out each character, but it will encourage the many completionists to do so.
Frequently Use Each Character
Some, but definitely not all, games benefit from encouraging the player to frequently switch between characters. This rewards generalists (rather than players who specialize in a single character/playstyle), and also prevents things from getting stale. It also (lightly) forces players to play characters they just don't like as much, whether that's for gameplay reasons or story/character reasons, and there are games that will be less enjoyable for doing that. But if you feel confident your game will be more fun when players are frequently using different characters and parties, there are several different mechanics you can use to do it.
A well-designed Elemental system can go a long way toward making each character rewarding to play. If each character only uses a single element, and each area is relatively predictable in the types of enemies it will throw at you, then there are characters that are better than normal for that area. For example, a Fire mage in a forest area. Maybe your fire mage isn't someone you would normally use, and maybe she's a little weaker than the rest of your party or you're not good with her skills, but when she's dealing double damage to nearly everything in sight, you'll likely still try her out (perhaps alongside one or two of your favorite characters).
Rest periods, like
@Ryisunique mentioned, can be effective, but they can also feel awkward or unfair, so it's best to keep a very light hand when using them as a designer. I find that a mandatory rest period tends to work best in a game where you have a limited number of in-game days to accomplish a goal (think
Recettear or
Harvest Moon). For example, I have a game where you can explore 1 dungeon (of your choice) each day, and after being used, a character needs 2 days rest before exploring another dungeon (so if the player wants to explore more dungeons tomorrow, he must choose different characters). In other games where you have unlimited time to do the things you want to do, I feel that using "well-rested" buffs for characters that have sat out of the last few battles is probably the best way to go. Straight-up forcing characters to sit out after 2 battles, for example, will break up the player's flow a lot, and will make the player feel like he's not allowed to use his favorite characters in the battles.
Limiting the characters' resources (and allowing the player to switch between characters at any point during a dungeon) can be a very natural and immersive way to encourage switching between characters. In many RPGs, resources like HP and MP are very easily restored - but in some of the best RPGs, they actually feel limited. If each character only has enough gas in the tank to take out three or four enemies, there are no (or few) items to restore those resources, and the player can be expected to encounter 30 enemies in a dungeon, then the player is going to need to cycle between eight to ten different characters in order to complete the dungeon. It doesn't require anything artificial, it feels like teamwork, and it definitely gets the players to switch up the character they're using frequently. If it suits your game design, you can very slowly regenerate each character's resources while they're not active.
Simply randomizing the party at the start of each standard encounter certainly does the job. (Some games do this only when touched from the back on the map by a monster, etc.)
All of these should be backed up by a system that awards partial (or even full) Experience Points to party members that aren't participating in battle, so that characters don't fall so far behind that none of these mechanics can justify using the weak and underleveled characters.
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One other thing I wanted to bring up:
Now I'm not saying, "make Dick a fire mage and Jane the ice warrior" because single-element characters always struck me as lazy design
Can I ask for a more full explanation of why you feel that Single-Element Characters are always (or even usually) lazy design,
@Aesica?
I'm surprised to hear that assertion, as I feel that Single-Element Characters are one of the handful of cases in which the Elements mechanic adds enough depth to justify its existence. Aside from encouraging a unique playstyle for each character (form follows function), assigning a single element to each character can be deliberately used to do one or more of these:
- spread the party's targets (Dick targets the ice enemy and Jane targets the fire enemy, rather than everyone teaming up and using their ice spell on the fire enemy)
- encourage the more frequent use of all characters (I'm going to need Dick in the forest area and I'm going to need Jane in the water dungeon)
- set up scenarios where Dick will be the support for Jane in one battle (against enemies weak to ice) whereas Jane will play second-fiddle to Dick's damage in the next (against enemies weak to fire).
All of these seem far less "lazy" design to me than "welp, looks like this enemy is weak to lightning, so everybody use your one lightning damage skill to damage it" (which significantly reduces the set of feasible moves for any given character at any given time to a very small number).