Tips & Pitfalls:
• TIP: When helping the reader get to know your MC, focus on what I like to call "universal human characteristics." This might sound boring up front, but what I mean is—focus on helping the reader understand your MC's fears, hopes, desires, etc. Desires can be especially endearing/sympathetic. If we know your character has always wanted to be a pilot, and that something is keeping them from that goal—bam—we're instantly rooting for them (and notice that will make for good future plot elements). These characters traits are what suck readers in because they can relate.
• PITFALL: Be careful not to give unnecessary information about your characters. If the MC has always wanted to be a pilot, but it has NO bearing on your story whatsoever, it will misguide the reader to care about something that doesn't get answered. Frustration will ensue.
• TIP: Save some information for later. Sometimes we feel like readers need to know everything about our characters the first time we meet them (Because how could a reader understand our story otherwise?). But really readers just need to know enough not to get lost. Trust them—readers are smart. Personally, when introducing new characters, I try to pick a few memorable things and leave the rest to the character's actions. Much more is said through how your characters interact than what you can say about them in an intro. Plus, saving info makes for great future plot twists. The beginning is for laying a foundation, not giving a full picture.
• PITFALL: The dreaded character soup. I tend to have large casts in my novels, and it can be tricky when introducing all the characters. The key is to do it gradually. Of course there isn't a right or wrong number, but personally I feel it gets confusing if you have more than three intros in one scene. Even then, they have to be very distinct characters so people don't lose track.
• TIP: Make characters stand out with specific, yet succinct, details. And not only in appearance, but in motives, beliefs, and attitudes. If your MC has three best friends and they all like cheerleading and watching the CW, I can guarantee you no one will really remember them. If you make one of them a raging environmentalist who constantly complains that they should have "green" pompoms and 100% cotton uniforms, I bet you every reader will remember her over the others.
• PITFALL: Don't get carried away in character building. This can be an issue for me, at least. While it's fun to have characters bantering back and forth and being funny, it's important that they are also always moving the plot forward. Characters shouldn't just be "hanging out." They need to be doing. Acting and reacting. Their conversations, interactions, and choices should contribute to a general forward motion and purpose.
Now, those are character specific tips/pitfalls, now I'll try to relate this to the plot information of last post. Every character has an arc just like the plot, and ideally the plot arc and MC arc are really quite intertwined. That's why your hero/heroine is the MC, after all. When talking about that "changing moment" and resulting choice all beginnings have, I'm talking about the MC.
But don't forget the other characters! If you want to breathe real life into your novel, take a look at those other characters. Do they exist for the sake of the MC alone? Are they plot devices? Or do they have their own lives? Of course their arcs aren't as large as the MC, but every character in a novel should be growing and making their own choices—not just choices that are convenient for the story, but real, logical choices. The events of your novel should have just as much impact on those characters (and their relationships with each other), and that impact should vary as much as people do.
Let's take a quick look at Harry Potter again. How does the beginning change the characters? Of course Harry is empowered—he's a wizard and feels important, though probably apprehensive and curious, etc. The Dursleys? Well, they change, too. They continue their cruelty and hatred of him, but they are more frightened of Harry and that alters things—he eventually gets a real room and everything. That event not only changed Harry, but them as well.
The web of characters and their reactions, motives, and relationships can get pretty overwhelming to keep track of, especially as you go along in a story, but that's part of what makes a novel sing. Real live characters—not just a vibrant main character—bring a story to life. Establishing those characters throughout the beginning is essential to building a story that readers can get behind.