From 2016, when Gottlieb published his memoir, here's an interesting interview:

https://www.theguardian.com/books/2016/sep/27/robert-gottlie...

The essential quality of an editor, he told me, is sympathy. “You don’t take on books with which you do not have a sympathy,” he says early in our conversation. “Only trouble can arise if instead of wanting to make a book that you like even better than it is, you want to change it into something that it isn’t.” The most disastrous thing that can happen in an editing process is for an editor to insist on making the book their own.