What if I finally get a book deal--and then I disagree with my editor?

I'm concerned that my editor/publisher will love my book, sign me, but then push to change things I'm not willing to change. Naturally, I want to be open to anything that makes the book better, but if I put my foot down strongly about a particular detail (or a couple of them), will that get me labeled as a difficult author?

This is unlikely for a number of reasons:

1. You're working with an agent of your choice--who, after all of your interrogation methods, seems to be on the same page you are with your vision for the work.
2. You were completely honest with your agent about how you see your work. 
3. This agent sent your book to editors who are likely to approve of this vision.
4. These editors saw a very good example of what you're envisioning--either your manuscript or your proposal.
5. Before signing you, these editors will have had a talk with your agent about proposed changes. You and your agent will discuss them.
6. If anything goes wrong, your agent is there to help. Yet another reason it's good to have an agent.
7. Most editors recognize that this is your book. They want you to be happy. 

Now, let's break down what an editor or agent is asking for with changes:

Let's say you're writing a murder mystery, and your agent/editor says, "Hmm, no, I think it should be Johnny who's the murderer."

Now, you like Johnny. He's mild-mannered, sweet, and he just didn't do it. 

You do not want to make Johnny the murderer. 

What is your agent/editor really saying? Probably something like one of the following:

1. This is too predictable.
2. Your current murderer isn't as interesting as Johnny.
3. Johnny's too boring. Make him more interesting.
4. I don't like your murderer's current motive. Make the motive for the murder different. 
5. It's more interesting if the murderer is someone who seems like an otherwise nice guy. 

Now. If you can figure out which of these it is--and that can be done by speaking with your agent/editor and gathering more information--you'll see that your agent/editor isn't really saying Johnny is a murderer. They're saying, "I see another issue, and I think this is the easiest way to solve it." 

They may or may not realize it, but if you can pinpoint what they actually want, then you can come up with alternatives that work for all of you. You do have to be gracious about it, of course. Your agent will help. 

But, again, drastic changes are something the editor will most likely know ahead of time--especially if you're writing fiction, since he/she will have seen your full manuscript before deciding. It's unlikely he/she will suddenly have an epiphany and decide to change your killer. 

If there is a major disagreement, your agent is always there to help with this process. Even if your agent disagrees with you (and agrees with the editor), he/she will be there to make sure someone comes up with a solution that works for everyone. 

We agents see stuff like this all the time. Remember Swordfish Shirt Guy? As you can imagine, he wanted some unusual things in his contract and his book. But it's worked out just fine.