What rehearsal techniques could you use to explore the relationship between two characters?

An insightful exercise you could consider would be status cards. After asking the cast to move freely around the space in character, you ask each to choose an imaginary card from a deck which signifies their status, with ace representing the lowest status and the King representing the highest). Once 'activated' the status should alter their posture, eye contact and proxemics to other cast members. Ask them to play around with different cards. Once comfortable try incorperating some off-text dialogue and then evenually ask them to identify what their status in the deck should be. This can be a cast discussion as no two people should have the same number. Once this is established, each characters status and power within the play is more tangible.