How can I develop a character from a text-based play?

To develop a character, first look closely at the information given to you in the text. There may even be a character description with their age, their relationships etc - if so, start there. If not, analyse the character's lines. Ask questions like how do they present themselves? What are their relationships? What are their motivations? It can be useful to do some wider research. If the play is set in a particular location or period of history, some research into that can contextualise the character and help you to understand the world they live in.
After that, you just need to use your imagination to really flesh out the character and fill in any gaps. How old are they? What do they look like? What do they think of the other characters? What do they want? What do they do for fun? It can be useful to try to answer these sort of questionnaire questions in character by "hotseating" to really get to know the details.