I know, it sounds like Acting 101, right? But I ask this question a lot, and you would be shocked at how often it is not clear.
If you have to think about it, then there is a problem. How can you know who it is and be reacting to in the moment if you cannot answer that question? It must be on the tip of your tongue, right there in front of you. Literally. With details. If you are speaking to your father in the side or scene, then you need to have sorted through that history. Is the background strong or weak? What baggage are you carrying around related to that relationship? It will have a bearing on the choices that you make. Be absolutely sure you covered everything you need to truthfully engage in that world.
Remember: who you are talking to needs to be right there, front and center.