If you need a cast of characters, there's one basic trick to it: never give two characters the same personality. Even if they're twins or triplets, they'll be different.
Really, it's that simple. I'll say it again: NEVER give two characters the same personality.
Oh, and one more important thing: Give EVERY character a goal. Even the minor ones, even the background extras--give all of them something to work for. Now, these goals don't have to be explicitly mentioned in the story, but keep them in mind, and your characters will automatically become more 3D.
Here's an example. Your main character is going to a fancypants party, so he needs to get his shoes shined.
Situation #1: He goes and gets his shoes shined. The shoeshiner is mentioned only as crouching down and whipping a rag across the main character's shoes until they are polished like a mirror.
Situation #2: The shoeshiner's goal could be to strike it big as a singer, so he starts belting out show-tunes as he shines the shoes, calming our protagonist's nerves a bit.
Which situation is more interesting? (Hint: #2) Now ask yourself why. In the first situation, the shoe shiner is just there, just like a fog hanging around the story, not really interesting or prominent. But in situation #2, he has a goal. Do you notice how he becomes more prominent in our minds even if he's only mentioned for a line or two in the story? He becomes more developed, just by having a goal, a motive, something to drive them to do what they do, and it makes the entire story more vivid and real.
Now, back to your cast of characters. How do you form it? While you can just write down different personality types, throw them into a hat, and blindly pick out a few, there is a very systematic way you can create the perfect blend of personalities that will stimulate your story and keep it interesting.
Basically, each major character (and even some minor ones) should fullfil certain roles. Here are the 16 roles to fill. There should be a character who:
1) PURSUEs the goal and ... 2) one who AVOIDs the goal.
3) one who HELPs someone's efforts and ... 4) one who HINDERs someone's efforts
5) one who tries to get someone to CONSIDER a course of action and ...6) one who tries to get someone to RECONSIDER a course of action
7) one who seeks a course or explanation that is LOGICally satisfying and ...8) one who seeks a course or explanation that FEELs emotionally fulfilling
9) one who exhibits self-CONTROL (focuses on one task or area to the exclusion of everything else) and...10) one who appears UNCONTROLled (tries to juggle or reacts to many things at once)
11) one who makes an appeal to CONSCIENCE and ...12) one who makes an appeal to TEMPTATION
13) one who SUPPORTs (speaks in favour of) any effort and ...14) one who OPPOSEs (speaks out against) any effort
15) one who expresses FAITH (confidence something is true, despite lack of proof) and ...16) one who expresses DISBELIEF (confidence something is false, despite lack of proof)
(This is quoted from http://www.how-to-write-a-book-now.com/archetypal-characters.html It comes from a writing theory called Dramatica, which you can Google. It's really helpful in planning your story, so it's worth a look.)
Don't think you need 16 different characters. A single character could fullfil several of these roles, but never have a character be two of the opposing ones such as self-control and uncontrol, or support and opposes. Even if they're conflicted, make them lean heavily toward one or the other. A character could, on the other hand, be logical, self-controlled, and opposes any effort. Mix and match these to come up with a unique set of characters.
These opposites create depth in the story. Look at Harry Potter as an example. Hermione would scold Harry for not studying, but Ron would tell her to chill. Hermione was the voice of logic and reason, whereas Harry did a lot of impulsive and stupid things. Hagrid was very emotional, Snape was emotionless. There were very strict professors, then there were light-hearted and fun ones.
Opposites create conflict, and conflict drives the plot. Conflict is the reason stories are interesting, so do all you can to create as much conflict as possible. If you have one character who is very hot-headed, have their best friend be very calm and laid back. If one is very childish and aloof, make another one mature and responsible. One character could always meticulously plan everything out, but their friend just rushes in and plans on the fly. BALANCE out the traits. Do not have an entire cast be hot-headed and emotional. Yes, this will ultimately create conflict, but the characters themselves will all be muddled together. In our minds, they'll all look the same, which is very bad. You want your characters to be very distinct from one another, and these opposing personality traits are the best way to do that apart from physical appearance (which, remember, should mirror their personality as closely as possible).
I can give you examples from SpongeBob: SpongeBob is very happy and fun-loving and immature, but Squidward is dull and gray and mature. Mr. Krabs is really cheap and money-minded, but his daughter, Pearl, has the typical teenager mindset with shopping and friends and parties. Patrick is an idiot, and Sandy is a scientist.
In the movie, The Avengers: Thor is hot-headed, but Bruce Banner is calm and mousy. Tony Stark is a genius and doesn't take orders from anyone, Steve Rogers barely knows what electricity is and believes he has to follow orders and give due respect to his overseeing officers.
Look at your favorite book, TV show, or movie, and you'll find characters with a plethora of opposing traits. The possibilities are endless, so find the traits that work for the story you want to tell. Once you've got a basic set of opposing traits you'd like to work with, now you should fine-tune each character's personality.
I'm sure many of you have heard of the Jung and Briggs Myers personality test. (Google it if you don't know about it!) I'm not sure if it's accurate, but what you should take away from it are the personality profiles. They are really awesome and detailed, and they can be an invaluable asset to you when creating characters. In the External Link to a wonderful website that goes into GREAT DETAIL about each personality type. You absolutely MUST check this out.