We often hear that the actor's considering and playing of 'relationships' is extremely important in any scene. That is, the actor, as the character, must be aware of, and be prepared to enact, a set of feelings and behaviors logical to past experience with the other characters in the scene: be it sisters, brothers, friends or lovers. Acting is 'real life' and therefore the actor must respond emotionally differently and specifically, to people based on the relationship we have had with those other people in their story lives; in Shakespeare's terms, good acting is a "mirror" held up the "nature. Characters in drama must behave accordingly.
For example: I fight differently with a brother as opposed to a stranger; I love a man that I've known for sixteen years differently than someone I've just met; I disagree with a boss differently than I disagree with an old friend. The quality and quantity of the emotional life of a character is dependent to the past experienced 'relationship'. In that sense, present feelings and attitudes that arise in a scene are reflections of the past.
However, their is a 'present relationship'
that is often distinct from (albeit interwoven with) the past relationship: it is based on what I want NOW from someone in the scene. The character 'relates' to the other person in the scene because he need something from that person. For example: I may in general hate my father because of what he did to me in the past, but my present attitudes in the scene NOW will also reflect the fact that, in the scene, I've come to borrow ten thousands dollars from him to feed my kids; and when the old man pulls out a checkbook, writes the number one followed by four zeros and hands it to me, gratitude, not to mention perhaps bewilderment, confusion and even love, may trump hate.
A good actor must not consider and restrict himself exclusively to old 'relationship' patterns or emotions in a scene. He must be prepared for and allow for new feelings to arise, present
and often unexpected feelings, due to the working out of the 'present relationship', which fundamentally is: "What do I want NOW from the other person in the scene; and how do I feel toward them based not only on our past relationship but also on the 'present relationship' (primarily what I am seeking from the other person).
There are in any scene two relationships
between characters, one past and one present, co-mingling and woven together. And the good actor must always factor both in to the emotional rendering of a scene.