Ed Lopat would tell pitchers he was working with, "Never the same pitch, never the same place, never the same speed"---and he often would have to tell them this more than once. He told me that this was particularly true when you're facing a batter for the third time in a game. Let's say you got Joe Zilch out on a curve ball his first two times at bat; but now he's coming to bat for the third time in a game and you can be sure he'll be looking for that pitch. He explained that it's important to discombooberate the hitter in every way possible, and that means move the ball around, high, low, inside, outside, and change speeds. Especially the last-named. Don't ever let the batter sit on a particular pitch, and stay out of his wheelhouse.
When he was working with me (and what an incredible pitching coach he was!) he would, for example, talk to me about the third time a batter came to the plate. "Watch him," he would say. "What is he doing up there at bat? Is he shifting his feet, or moving closer to the front of the plate or hitting with his foot in the bucket? Is he choking up on the bat, as if indicating he might bunt---or is he looking for a pitch he might try to drive to the opposite field?" What he was doing was getting me to think about these hitters and how best to pitch to them. One thing for sure---I didn't have a fast ball to speak of, and so I had to go to my breaking stuff---and believe me, I had a lot of breaking stuff, and he would show me how best to use it.