I generally have a small group of people playing D&D at any one time. However to get the party dynamics right, and to cover all useful / necessary skills we could often use more characters than there are players.
In what situations is it acceptable for players to play more than one character each at the same time in the same game?
In what situations is this a hindrance to the immersion in the game world or prevents character development?
It's probably useful to distinguish between playing more than one character sequentially and more than one character at once. I'm assuming from your question that you mean at once.
I'd say it's acceptable any time your group is comfortable with it. Some games explicitly call for this: Apocalypse World gives players the ability to play multiple PCs as part of advancement, which I think is very clever: new players won't be able to do it, but once you've played the game long enough to really get a feel for it, you have the option. Early D&D also more or less assumed you'd have henchmen and so forth, which is not exactly the same thing, but it might have the seeds of useful ideas.
What if, rather than making the extra characters full PCs, you used some sort of companion rules? Strip down the characters to their essentials, explicitly assume that they won't get as much roleplaying attention, and so forth.
Two specific ideas in that regard: one, reserve the right as the GM to control companions if you ever want to. I would use this as a mechanism for the companion to say no to orders, mostly, and I'd use it fairly rarely -- think of it as a morale thing. "Dude, you ordered Bob to go down there and he never came back."
Two, make them pool characters, so that different people run them at different sessions. That will have the specific effect of minimizing immersion in the companion characters, so make sure you want to do that as opposed to increasing identification.
I think no matter what you do, this is going to affect your immersion a bit, simply because you'll have to make decisions about more than one character. But focusing on the main PC should help some.
We tried, with decent positive level of success. In retrospect, however, we felt a decreased immersion.
In what situations is it acceptable for players to play more than one character each at the same time in the same game?
When you desperately need cannon fodder. Example: when playing a slaughter house module such as Return to the tomb of horrors in d&d, multiple characters per person is an important nice-to-have. People grow an affection to their character, and if the module makes resurrection hard or impossible to achieve, having two characters helps in many different ways:
In what situations is this a hindrance to the immersion in the game world or prevents character development?
If you have a campaign where interpretation is primary, and chances to die with no resurrections available are low, having multiple characters is not a good solution. The only exception is the case of a player choosing to have twins as characters. That makes things nice and unusual.
Players running more than one character at once is almost always going to mess with immersion to some extent: it splits up the "I" out-of-game between several "I"s in-game. How badly depends on the particular players and the particular group dynamic; in some it might not noticeably impact immersion, but in most it'll give you at least some trouble.
That said, there are styles of gaming where immersion just isn't that important. If the game is mostly about dealing with challenges (either tactical, as in the case of D&D 4e, or strategic, as in the case of a classic OD&D megadungeon) then the players can have a lot of fun even without strongly identifying with their characters. Maybe more fun -- less direct identification means that it's funny rather than wrenching when someone falls in battle. It also gives each player more tactical options under their own control.
Likewise, if the players are focused more on the overall arc of the campaign and it's developing story than the experience of their particular character, giving them multiple characters to control can give them more tools with which to influence that story, and different angles to come at it with. Players who enjoy building and exploring characters as a writer would might also enjoy having several to explore at once, and to play off of each other.
If, however, getting deeply involved with a single character, and the deeper connection that creates between the players and the campaign, is important to you or your players, multiple characters is probably not the best solution to your problem. As others have said, making one character the "main" and treating the others as henchmen might be a good compromise. Giving the characters henchmen run by the DM might be another. However, you might also consider switching to a style of game where things like fully covering niches and having sufficient numbers in combat isn't as important, which might or might not require using another system.
When running earlier editions of D&D and its clones I usually have the players run two PCs a piece if there are less than four players present. There's an assumption built into those games that the PCs will use hirelings, which I and many other people find to be immersion breaking anyway as well as it being a bit of a pain to keep track of half a dozen hirelings.
The likelihood of PC death also makes running two PCs an attractive option, if you lose one you still have another to continue on with and don't have to take time out to roll up another PC.
In almost any other game I'm pretty adamant about one PC per character, unless the game system calls for group control of a bunch of characters or something similar.
I've played in campaigns in which I've ran a single player and others in which I've ran more than one. I didn't experience any less immersion running more than one character than I did with only one. Whether I was immersed depended on how lively of an environment is created by the Storyteller.
Still, I have had others who play tell me they do not like playing more than one character. They claim that it prevents them from getting into the roll.
I would say that it entirely depends on the players as to whether they could/should run more than one character. If the group is not interested in doing that or doesn't seem to be able to do it well, then I would add henchmen or NPCs to fill in gaps. Otherwise, let them play multiple characters if you're up for it. A good Storyteller shouldn't have an issue with immersion because players have more than one character.
Perhaps not directly relevant to the question, but still relating to one of the needs:
One method I've seen to fill out the party composition is that a DM might add an NPC to the mix. The NPC would most often remain in the background doing nothing noteworthy until a combat situation arises, or one of the PCs could request it to do a task that it's particularly well-suited for.
I've had this come up in one group I played with, where several players' attendance was particularly irregular. We'd captured one goblin rather early on in the campaign, who later came to be pretty friendly. After several encounters, the NPC was still around so our DM decided to make him into a full-blown character (Rogue) who would be a companion to help fill out our group.
This also has the added fringe benefit of having a "visitor's character" available at all times. Once in awhile, someone not normally in the group may want to join for just one night. Instead of having to draw up a whole new character and role-play them into the story, we just let them control the NPC.
Ars Magica suggests that every player has one mage, one companion, and a few grogs that they can play depending on the situation. Personally, I really liked that as it allowed for variation in play and characterisation of minor characters. It allows for "this is the right mix of people to take to adventure X".
We are just wrapping up a game where the players were each running two characters simultaneously. With only 4 characters, we were getting in too deep in some encounters... so we expanded the group. For the most part, I think it worked well. The more experienced players were able to keep their characters separate, have them argue / agree / disagree according to their own personalities. The less experienced players had more difficulty running characters as individuals. The biggest issue we had was a tendency toward metagaming.
So, really, I think it depends on what you're looking for in a game. If your modules are difficult and your players are really good, it's not impossible to run multiple characters successfully. On the other hand, if you really want immersion and want to avoid metagaming, even with good players I would recommend sticking to one character at a time.
In our next campaign, we will be back to controlling only one character each. With luck, this will get us back to the immersive "I" game without getting us all killed instantly!
Acceptable? Yes, depends on how you do it. One thing you(The GM) needs to watch out for is one player making deals between characters that they play that would go against one or either character's personality. Some game systems have something like this built in, i.e HackMaster 4th Edition has a whole NPC system that includes not only Hirelings and Henchmen but also Proteges and Sidekicks. In HM4 a Sidekick(you only ever get one) is someone with a friendly connection to your character, for example maybe your best friend from back home or school/training, and that character is never destined to become a Primary Character. This is the buddy that not only would bail you out of jail but is probably in jail with you sayin, "Dang that was an awesome night!" so therefore the Sidekick's morale/loyalty is a lot greater than nomral. The HackMaster 4th Edition "Protege" is a bit different Basically a Protege is a character that your current character is acting as a Mentor to, training them in the ways of being an adventurer. They don't have to accompany you but sometimes they do. You can expend some of your Experience Awards(Points) on them to help them gain levels, even if they aren't with you on the adventure. They can never get to be the same level as you, because then they wouldn't need you. Typically in our games, if a protege is along, they only gain half the experience as a Primary Character. The good thing about a Protege being along is that it is a good way to round out your party and it also gives you a spare character in case your Primary dies. Really, that's the essence of the Protege, that of an Heir.
In HM4 I've seen parties fleshed out with 2 or 3 proteges and a Hired NPC along to round out a weak party. Hired Fighters are pretty easy to run and so is a Hired "Healbot" Cleric. It all depends on how the GM and Players run it but as a GM, I'd advise being careful that the Players don't abuse alternate characters, i.e. they don't run them by their supposed personality/alignment.
