The easiest way to create conflict between characters is to pit their different traits against one another.
If your character is more of a "homebody," for instance, put them with someone adventurous. If they are highly educated, pair them with someone who's street smart. If they are superficial, put them with someone who is kind and humble and caring. Are they tidy? Force them to room with a slob. Shy? Pair them with someone who is outgoing.
Think in terms of opposites.
Someone who is affable and sensible is going to act and react differently from someone who is disagreeable and disruptive. An optimist and pessimist see the world in two completely disparate ways.
Pair the opposites together, and the conflict creates itself.
The tension builds naturally.
And this isn't just about pitting a "negative" trait against a "positive" trait. Some situations will call for street smarts over book learning. Some situations require disruption--for someone to act out to bring some new information to light. And not everyone responds to affability. Enter the character who is caustic and ruthless. They'll get the job done.
In terms of arcs, think about these traits as the characters work against each other. It's easy to see that where one leaves off, the other can pick up--in some cases, they may even become complementary.
Create a series of situations and watch these opposing characters naturally begin to influence one another, making room for their growth.
Maybe your main character *is* a bit of a homebody. Maybe he needs the influence of that outgoing, adventurous girl so that he can have some escapades of his own to remember fondly. Maybe the one chasing adventure needs to learn to settle somewhere--to stop running--to put down some roots. Maybe the slob needs the neat freak to show them what a nice space could look like.
And remember to show, not tell.
If a character is shy, how might that reflect in their behavior? Maybe they don't want to mention to the server that they were given the wrong order.
How "tidy" are we talking? Maybe their closet is organized by color and their towels are folded into thirds.
And don't feel boxed in with these traits. At some point that typically shy person is going to get sick of being offered the cheeseburger when she has repeatedly asked for the chicken sandwich. This is her chance to act out. And if the slob comes home to find his roommate's space dismantled, it's a clear cause for concern.
Fix your character's traits then subvert them, surprising us.
In sum, as you consider your character's traits, brainstorm how those traits can manifest or show themselves in your story, especially when they are forced to work with those who are not like them. Think about the ways in which they can act out of character. Think about who they are at the beginning of the story, and who they will be by the end.
In other words, make them as real and as complicated as you and I.
Make them human.
Be Brilliant!
~Katie~
