How can I train myself to think in "multiple personalities" so I can use these personalities for character development in writing?
Quick side note: Role Playing Games such as Dungeons and Dragons helped me out with this. Playing make believe, too.
Now the answer:
You need what many call a brief, although stating "brief" is just the name, being a rather longish document. In a character's brief, you write down their characteristics. A good thing to do is write down their traits (easily angered, quick to laugh, spontaneous), then when writing a scene, look back at these traits and check for incongruousness. You might find, or not. In any case, you don't have to "think" in multiple personalities. You give the characters traits. They take life and you follow them and their actions. Write what you think first, then check back on the character's traits and edit the scene.
You might have written the character as slow to anger in a scene, but given it (at the moment of fleshing out) a quick temper. You need to change accordingly, unless the character is growing out of their quick temper; in that case, you should take note appropriately in the place you have those traits written down. "She starts learning how to control her temper in this and that scenes".
This helps a lot to keep track of character believability, and also with character development.
One technique, as suggested by the author David Mitchell (who is known for his 1st person prose) is to write a whole bunch of autobiographical letters from your characters to you, the author.
You won't use these letters verbatim in your writing, but even if your character is narrated in the third person they will be valuable as a tool for getting in the mind of your character. Not only will you explore your character's bio (which is the traditional way of character exploration), but by writing the letters in the first person you'll also explore their language use, which in turn helps you understand their mentality and perspective on the world.
I've used this technique and can vouch for it. It's pretty effective, though requires a bit more time investment than the traditional character brief.
I've sometimes taken online personality tests, answering the questions as my characters would. I particularly like the Myers-Briggs version - there's one available here that works for me. The test itself can be helpful, as you have to really think about your characters in order to answer some of the questions, but the summary at the end is what I like the best. It classifies each character as one of sixteen personality types, and then reflects on how people of that type get along with others, view themselves and the world, etc.
It's not foolproof, but I find it helpful, and it's kind of fun, too!
Base the characters off amalgamations of people you know well. Or people that you have spent enough time with to know how they would react in various different situations.
Also make an effort to really pay attention to people. Rather than engaging yourself in conversations, observe them, noting:
Being an extreme introvert, this is how I have (sadly) always engaged in conversations. But I've suggested it as a writing exercise before and have heard good results.
I would go the other way round: any tried 'multiple personality' voice, which really does not resonate on paper once written, should be at least strongly questioned when revising your text. Only those passing this test are likely to work for the reader; the same applies to the relevant characters.
Moreover, to keep a proper balance in narration perspective, among characters and their voices, it helps to stick to the simple rule of using the third person whenever possible, as in Franzen ten 'serious rules':
Write in the third person unless a really distinctive first-person voice offers itself irresistibly.
I had same problem. My main character is affected of sort of DID (dissociative identity disorder). I made my way in a different path than other answerers.
1) Will be all of them dominate or you will have one main personality and other will be something like voices in your head?
2) Will your other personalities take control of a "body" and its actions?
These questions must be answered first, i think, because it is pin point task for you to write about it.
I personally tried to make other personalities by myself. It is akward, different. Imagine yourself that you have in head two other people and everything you do, every decission you make you consult with other people and their vote is same weighted as yours.
I consulted this with psychologist and read books about DID to have enough information about this.
Now i have main character with about 5 different personalities of him and in writing it looks like he is speaking not about him, but about them in himself. It is fun, but you need to be sure, when different personality will affect your characters actions.
Hope this will help.