If you're a novelist, you probably spend more time with your characters than with real people. In addition to putting their actions, words, and thoughts into prose, their needs, concerns, and tough choices probably occupy you mind all through the day and possibly in your dreams. If you're like me, they also talk to you, often at the most inopportune times.
I have no interest in devoting such a big chunk of my life to characters who repel or bore me. One of the secrets to productive writing is making the overall experience fun. If it is painful or unpleasant, it becomes something to avoid. And characters play a big part in the experience.
Naturally, the main character should be empathetic, even likable. The MC is the one who keeps the reader engaged. But, for the writer, all important characters should be engaging, even the villains. In most cases, even for pantsers, just telling the story isn't enough to make that connection. More is needed.
Some writers go through a great deal of character building and that becomes a basis for a relationship. To me, the classic fill-in-the-blanks exercises are sterile. Knowing a character in abstract terms isn't enough. I interview my characters, and often ask open-ended, even quirky questions. That does a lot of the job for me, but sometimes I need more. Here are a few other things I've tried:
Most embarrassing moment -- I think this is a sure way to build sympathy. We have all suffered from embarrassment, and the experience can be vivid and memorable. Now, what embarrasses me may not to embarrass some of my characters, so just putting a character through my embarrassing moments won't always work. But failed attempts usually point me toward something that will expose and distress even the most hardened characters.
Listening to the voice -- Sound is important to me. In my head, I hear the diction, intonation, timbre, and delivery of each character when I am involved with my work in progress. Almost always, these voices come to me spontaneously, but occasionally, I've had to keep my ears open as I've watched TV, sampled YouTube, or eavesdropped a the mall. I suspect some writers shanghai a voice they've heard, and, if the works for you, great. What happens for me is, after experiencing many voices, the character seems to select bits and pieces as his or her own, and I'll get the voice in a quiet moment, without choosing it.
Working together -- For me, the best way to get to know someone is to collaborate on a project. Many of my projects (like this blog) are in my head already, and it's easy for me to get a character to join in. Where they participate, how they need to be managed, and what insights they have reveal them in new ways. (For the less intellectual characters, I may need to imagine help with something physical like mowing a lawn or making a repair.)
Asking advice -- One thing that can get in the way of writing is having a worry or concern. Often, the problem will be interpersonal. Asking advice from characters can help me explore solutions and enlighten me about the character at the same time. Of course, if the character suggests a solution that is criminal, it is best not to put this into practice in the real world.
Introducing to a friend -- I like this best. I don't introduce my characters literally. It all happens in my imagination, so the friends don't need to be in recent contact or even among the living. The important thing is leveraging what I know about friends and how they're likely to react to elicit moments that tell me more about my characters. Often, this is so successful, I'll introduce the characters to a number of friends.
Overall, it's about inviting characters into my real life in some way that is vivid and compelling to me. Not every approach works for every character, and some of these can lead to unpleasant experiences (which can be useful later on). If they are dull through and through, I work to remove them from the story or minimize their participation. They fail the casting call. But, if they are unpleasant or repellant, I'm apt to keep trying exercises until I find one dimension that reveals what I need to empathize with the character, and that is enough.