Before I start, I have to say I wish more of the questions were like this. This is red meat for gamers who like to argue about their hobby.
My answer to this is going to center around the non-mechanical aspects, as I feel like the functionality of a character and the interface of mechanical and non-mechanical character traits is a different issue altogether.
BackstoryFirst, a good character has to have a backstory.
It's a common trope that poorly made characters have no history. They sprang from the earth, fully formed and clad in adventuring gear. Some games don't really need more than that; Fi-tor the Fighter doesn't need a backstory if all he's doing is descending into an endless dungeon to fight monsters until he dies.
|Unless you're literally playing Athena, then springing forth fully-formed is probably okay.|
It doesn't have to be War and Peace, it just needs to be enough to understand what motivates the character and why. What benefit is all of this motivation?
A good character also needs goals.
This starts with justifying why your character is even on the adventure in the first place. Frodo's life was not one long build-up leading to Bilbo handing him the One Ring, but the events of his life made it plausible for Frodo to accept the quest to take it to Mordor. The well-written character will follow the adventure for reasons besides "I'm good and the bad guy is evil" or "All my friends are doing it."
|Some exceptions may apply.|
It goes beyond that, though. Most people have some interests outside of their profession. Presumably, your character should have things that motivate him besides completing his quest. Do they have hobbies? Are they collectors or trophy hunters? Are they part of an organization? Are they religious? Do they have any feelings about the local government?
This doesn't have to be an extensive or comprehensive list, but having a few things that the character wants out of the game will go a long way towards making him three-dimensional.
To really shine at the table, however, a character needs personality, things that make him unique to the other characters at the table and those you've played as well.
A lot of people like to do voices and accents for this, which is fine. A lot of us are only mediocre at best at this sort of thing, so it's probably best to skip it unless you do some practicing ahead of time.
Still, distinguishing the character at the table goes beyond just the voice. but also the mannerisms, the tone, and other affectations. This can be something incorporated into the way the character speaks, but it can also branch into the way you describe the character's actions.
For example, perhaps the character is shy. This could be voiced by a slight studder and squeamishness when speaking with strangers. When possible, perhaps the character chooses to stand behind other characters, being out of direct line of sight. They probably don't make a lot of eye contact, either.
|Just don't do catchphrases. The other players will try to murder you.|
None of these things in isolation may seem like much, but in concert they can take a bland, boring character and turn him into something memorable and vibrant.