Because of my stutter, one of my biggest pet peeves is someone who either finishes my sentences for me, or interrupts me thinking they know what I'm going to say. My significant other does this all the time to me. For a while I gave her the benefit of the doubt, but after it persisted, I sat her down and had a talk. First think I told her was that interrupting or finishing my sentences is a huge pet peeve of mine. This could be the first step when talking to your father; look him in the eyes and calmly say, "If you could please let me finish my sentences, I would really appreciate it, it's a pet peeve of mine." I've found people are more apt to change their behavior if they know your pet peeves. Second, I told my significant other that I don't feel respected when I'm interrupted and that because of my stutter, it takes longer for me to have a conversation. Stress that patience is key, and hopefully your Dad will try and respect your wishes. The most important thing is to have patience yourself. If you get mad, he will only get madder. This might take a long time, but keep reminding him, and having open, honest conversations will build a trust/mutual understanding between you two. For my situation, my SO is working on their interruptions. It can be very frustrating at times, but I know it will all be better with time and practice.