The best characters in literature go through change. They start a story in one emotional or mental state and end someplace very different. They may want something, or believe they want something, at the start of a story, then by the end want something very different. In writing, we call it character growth, or a character's arc.
When you are developing a character for any roleplaying game, keep in mind what emotional state they are starting in and how they may change by the time the campaign or story is over. You don't have to know exactly what that change will be, but giving a character the opportunity to grow adds depth and freshness to Player Characters (PCs), particularly in long campaigns. The change may be purely roleplaying, like hating a particular race, culture or non-player character (NPC), then learning to trust them, or it may manifest itself as a game-related change, like adding levels in a new class. Or it could be both.
My most recent Pathfinder character was a dwarven ranger (trophy hunter) by the name of Hark. His father had invented one of our campaign's first true rifles. Unfortunately, his father had been assassinated by a bugbear before revealing to Harsk how to activate the weapon's magical properties. Harsk wanted two things, find the bugbear that murdered his father and uncover the mysteries of the his father's greatest gift--understandable but not very deep motivations. My friend Victor had chosen to play a particularly disturbing witch named Onyx. When Harsk was first introduced to Onyx, I knew Harsk would hate and fear her. I also decided that over time he would come to admire and even look up to her. He would eventually look up to her as a teacher in order to increase his own alchemical skills, then turn to her for help in his most personal quest, uncovering the powers of his father's rifle and the secret behind his murder. That's when I realized what Harsk really wanted--a mentor, someone to finish the lessons his father had started. As much of a brusk, dirty, field-hardened warrior as he was (Charisma 5), what he really wanted was someone to trust.
I didn't share this motivation with the other players, I simply let Harsk's attitude inform my roleplaying. Six or seven games in, Victor decided he wanted to try something new. Our GM (not telling the rest of us) decided that Onyx would be bribed by the bugbear that killed Harsk's father to betray us. It was a perfect solution that allowed Victor's new character to step in while turning a former ally into an adversary.
At the end of our previous game, Onyx and Harsk were scouting out a bugbear camp they suspected housed the assassin. The following game opened with Onyx muttering a reluctant "I'm so sorry," just before a dozen bugbear's burst into the clearing and pounded Harsk unconscious. Harsk was far more than physically betrayed. As soon as I heard the words come out of Onyx's mouth, I felt how Harsk would feel. He had just started to trust her. Over the course of several games, she was becoming the only real friend he'd ever had. I loved it! Even though Harsk's heartbreak happened only in my own mind and we didn't get a chance to play the fallout (the game ended with a brutal TPK*), it made my personal game far more interesting.
One of the best recent examples of "character development vs game mechanics" I've heard came from friend and brilliant cartographer, Christopher West. Chris told me about a Pathfinder character he ran that was a 3rd level Cavalier/1st level Witch. As I remember it the character had died, but been saved by some unknown, dark force. The character didn't want to accept that tainted magic had retrieved his soul and reanimated his body, he only knew that an eerie raven followed him everywhere, whispering to him, and from time to time twisted magic would manifest around him. Chris controlled when his character 'cast' spells or invoked hexes (usually in selfdefense), but his character didn't understand what was happening to him. His character started in a very specific mental state and I can see how, over a campaign, he might end someplace very different. Would he accept this magic? Would he find that he owed a demonic force something and continue to deny his building power? Would he realize that the magic is a part of him and not as evil as he first believed.
From a game-balance standpoint, a cavalier/witch is a bizarre multiclass with several number-crunching limitations. As a GM, though, I would LOVE to see that character's arc and would do whatever I could to make it so those limitations didn't unbalance the character against the rest of the party.
So the next time you're developing a character don't let them be static. Give them space to grow and change, then see where they take you. It might be a place far more interesting than you can imagine on your own.
* = Total Party Kill. When the rest of the party tried to save Harsk, it went terribly wrong.