You must have watched a movie or read a book, and the character made you cry, angry, curse at the screen, or you instantly liked them the second they appeared. One of the reasons is because the character was three dimensional "believable."
So what is a 3D character?
In most cases, you do not want your character to be the same as the others; he/she should usually be the complete opposite of the antagonist.
Build conflict between the characters; this includes friends, allies, etc. I'll dive into this in another post.
So first of all, this is an essential starting point list for any believable character.
Place of birth
There are more, but I'll focus on these.
Job; Well, this can say a lot about a person. Let's say he is a police detective, right away we know that he is likely intelligent, seasoned, plenty of initiative, likely proficient with firearms and self-defense. Feared, respected, covert, possibly cunning, excellent problem-solving skills, and streetwise, etc. All this, simply by his job.
Background; likely his history, what school he went too, why he wanted to be a cop, his achievements, relationships. Doe's he have kids, a dog, problems at school, did he go to school, etc, etc, etc. Quite an extensive list for this one. If you can write a page or two, just covering their background, then great.
Skills; This is a fun one. You can get creative, and usually, the skills should have something to do with their background, why they have these skills. Skills are typically not the character's job; skills are something special he can do.
Race; What colour is your character. Sometimes not necessary, though, for some stories, it can play essential factors; it can also help with the character's background.
Age; Well, if he is 90, then its unlikely he will be able to sprint after fleeing criminals. He is a detective, so likely 30 plus, and unlikely he would be 15.
Gender; Rather obvious, for example, if your story is based upon truth, then you would not write your character is a female royal marines commando, as RM only recruits men.
Parents; Plays into the background too, were they rich, poor, abusive, divorced, etc? All can affect your character's personality.
Walls; What is stopping your character from getting the task done? What must they overcome? How will he get there?
Birthplace; Goes with background; If fact-based, then if your character is British, he would not be in the Chinese army.
Personality; Another fun one, is he shy, aggressive, depressed, funny, goes together with his background, and sometimes his job.
Goals; What is he after, what drives him, etc?
So, if you write a character to make up the numbers, or for one task, forget it, don't bother. If your movie has five main characters, why not make it three instead. It's less to focus on, and it makes the characters stronger by bringing more depth to them by combing two or more characters' skills, backgrounds, etc.