Every character has a story. As a playwright, you task is to tell that story in a convincing manner without directly telling the story. What do I mean by that? Well, think of it this way, you have to describe your character's past and future goals by only allowing them to speak in the present. Flashbacks or flash forwards are not common and every time they happen it seems the audience draws a direct correlation to It's a Wonderful Life or A Christmas Carol. Holiday films/plays aside, it is advisable to stick to the present for your dialog. Also remember that the two works previously mentioned were works of literature before they were works of theatre.

Your characters need a life. You have the ability to provide them with one. Here are the three elements to create for every character:

1. A back story. Create a plausible life for your character that defines their present circumstances, mannerisms, language, and world view.
2. A future goal. This is normally defined by the back story. The character want what they want because of what the have done/seen/heard/lived up to the moment we see them in the play. This is the driving force of the actions they take toward the goal. It gives you fodder for conflicts and obstacles which lead to the climaxes.
3. Their present. The characters must live and speak in the present for us. Even in the aforementioned works, we only are allowed to see the past and future through the eyes of the characters.  The character's actions and dialog are defined by the actions of the past and the future goals of the character.  Your writing is reflective of this. The formation of the character only happens for the audience in what the character says or what is said about the character by other characters.