Characters operate within the constraints of their circumstances. Two characters that may otherwise be friends may be turned against each other by mutually exclusive objectives or hostile circumstances. For example, Mercutio and Thibeault in "Romeo and Juliet" display similar values and would perhaps have gotten along, were they not allied with warring families.
George and Lenny in Of Mice and Men also display an interesting relationship. They are very different, but in need of one another in unique ways. They are both in difficult economic circumstances, and have disproportionate advantages (George's intellect, Lenny's strength).
The circumstances characters are in dictates, to an extent, the relationship between characters, because it dictates whether the characters offer an facilitation or obstacle to one another's objectives. Even within a play, a change in the situation and thus the calculus of the characters can change the dynamic between them.
Contrast is the obvious difference between two or more different objects when they are put together or compared. A contrast is formed when things, people, places or concepts are put together or compared in order to show their differences.
