It goes without saying that the final responsibility for pitching is with the pitcher. But the catcher can help a lot. He can switch to another pitch if one isn't working. He can switch locations is one isn't working. He can alter sequences. He plays a role in pace and rhythm. Sometimes, if you shift away from a pitch, it will be better when you come back to it. Same with sequences, especially the third time through an order.
He can try to keep a pitcher in focus. 
There are games in which nothing can be done -- like Bard's last start and a few others, like Beckett's first start, when he insisted upon throwing belt high FB's with no movement or hanging curves. Beckett was slipping into those habits in his last start, but Salty got him away from them -- with Beckett's cooperation, of course.
Some catchers have a better "feel" for what is going on than do others. Some are better psychologists than others. Some have a stronger presence than others behind the plate.
None of this absolves the pitcher of final responsibility. But all of it is involved in the "battery."