Yes, very common. Two words of advice (from my speech therapist, actually): 1) It's absolutely normal for everyone to think about "important" sentences even hours or days in advance. When you realize that you are going through that sentence in your head many times, try to stop it: you know that your speech is not affected by how many times you "rehearse" mentally! :-) 2) When you find yourself stuttering a sentence over and over in your head just stop, relax, and pause until you feel that you can start think of that sentence without stuttering. When you are sure that you can do it, think about it! Think about saying that sentence in a completely relaxed way, with no stuttering, but take as much time as you need before. Also breathe. Compliments: you just activated a virtuous circle. The brain is learning mainly from going through the same thing over and over, and the repeating (google for "mirror neurons"), you just taught your brain how you want it to behave. Also, "good behaviors" are recognized much faster than "bad ones". Works for me, your mileage may vary! Good luck for your interview!