The Motto is "Never Split the Party" for a Reason
One of my players has a great blog about his experiences and thoughts regarding D&D and recently he wrote an entry about something that happened in one of our sessions. Due to how the session evolved, he felt that his character would do something completely different from the rest of the party. Basically, they were accompanying the king and queen back to their capital city and it was clear that the king's chief advisor was not quite right. Mike's character, Skamos, as someone who is generally leery of all authority, understandably wanted to follow around the advisor, but the rest of the party (and to some degree, the story) wanted to be in another location. This left me with a hard dilemma. He could go off and do what he wanted, realistically meaning he could be out of the game for the rest of the session at least. Alternatively, I could find a way to get him to the rest of the action. Since I want everyone at my table to have fun, I did the latter. Someone wearing a cloak just like the character he was following and of the same height and build left the inn where the king and queen were staying and traveled to the tavern where the rest of the group was hanging out, having a few drinks and flirting with the locals. I felt terrible doing it, but at least everyone was together right?
Unfortunately, it turns out it was just the beginning of the story just not flowing in the way I had hoped. The character also didn't feel like sharing in the libations. Completely understandable, since well, they were adulterated with goodnight tincture, but that wasn't something the characters would have known. This meant that when they went outside to get some fresh air, his character was the only one standing while the rest of them passed out. And when the bad guys who were there to kidnap them asked him to give up his weapons, he refused to do so. Which left me with the choices of attempting to (and probably actually) killing his character, leaving him behind or having the NPCs allow him to go with them with his weapons. In the end, I chose the last option, but I just felt like a complete failure at this point. As a person playing the game, I know that none of the other players want to sit out a session. Also, I really try to not railroad my players and had their characters all decided to call it a night and never go out drinking, I would have come up with a completely different story. But I was having a really hard time figuring out what to do when only some wanted to go in a particular direction.
Days, well really a few weeks later, I know that the failure wasn't completely my fault. Sure, there are things I could have done better. Having a better understanding of the motivations of my players' characters would definitely help. Finding other ways to get the unsuspecting PC unconscious would help too. But in the end, the burden isn't the DM's alone. Just like the NPCs can't always act the way they "naturally" would, PCs need to have the same flexibility. Otherwise, it can be a bit not fun and, at that point, why keep playing? And if a player is particularly adamant about not going with the rest of the group, as much as it might hurt to do it, it might be better to let them go. I do like his idea of letting the other players play the other NPCs in the fight, although I would be concerned that they might be a little easy on him since they might need him later.
All that said, I think there are times when the party could be split. Ameron on the Dungeon's Master has a great article about how splitting the party worked really well in a game he was in. And having a Leverage-style game would be really cool now and then. However, unless it's one of those times, please do you DM a small favor and don't split the party.