How do you write a scene where a character is talking to himself or is talking to an object in a book?

There’s no standard way to do this I’m aware of. You can, depending on the style of the book and your own writing style:

Write it as normal dialogue. Rather than simply saying “said,” you might use dialogue tags like “answered himself.”

Write part of the dialogue as normal dialogue, especially if the character is literally talking to himself (as in talking out loud), and write the answers in italics.

Write the whole thing as dialogue, but using italics rather than quotation marks.

There are probably other ways to handle it as well.

I personally favor the second option: a back and forth dialogue, but with parts in italics. This is especially useful if the character is talking aloud to himself but only thinking the responses.

That’s how my Talespinner and I handled it in the literary novel we’re writing, with the character Lazlo, who frequently talks to himself:

“Do I have any appointments or visitors scheduled for today?”

“Master Shakes isn’t scheduled to pick up his commission until tomorrow afternoon. You did say you would attend Master Farra’s soiree this evening, however.”

“Good heavens, did I? Why would I agree to that? Eat cold food, drink the cheapest wine the host thinks she can get away with, and gab, gab, gab.” He sniffed at the bread. “Please inform her I will not be attending.”

“Oh, I’m certain I can’t do that, Master Lazlow. You already told her you’d be there.”

“I did? Why would I want to go to something like that?”

“You were undoubtedly invited for Master Farra’s benefit, not yours.”

“Why? She’s perfectly competent. She wouldn’t have made master otherwise.”

“Oh, I don’t question her competence, Master Lazlo, not for a moment. But she doesn’t have your name. It gives her quite some influence to be able say, ‘Master Keschel attends my parties.’ She was delighted when you said yes, which is why you can hardly back out now. You can always use more allies in the Guild, you know.”

He sniffed at the bread again, then bit into it. “If you say so, Margaret.,” he said around the mouthful. “Begone. I have work to do.”

Her warm chuckle trailed behind her as she made for the door. “As you always do, Master Lazlo. I will send Jacob to fetch you at eight o’clock. Do please mind the bell, and try to be in good spirits, or at least as good as you ever are.”

“Yes, yes, good spirits.” He waved her away.

By the time she’d left, his mind was already back to picking at the perplexing puzzle of the bird. A perverse temptation urged him to disassemble it, despite its protests. After all, he had no way to know that it was in fact alive, merely its word that it was. And it might be made to say that even if it wasn’t, right? It seemed unlikely that a mechanical thing might be brought to life. A mechanical thing might easily be designed to say whatever its maker wanted it to say, even things that weren’t true, which could trivially extend to claiming an inner awareness that didn’t, in fact, exist. He had no reason to assume that what it said was so, and plenty of reason to believe that a mechanical thing was simply a thing.

And yet, some contrary voice within him objected, what if you’re wrong? A toy brought to life—that would be a miraculous thing indeed. Be a shame to destroy such a thing simply because you saw it as a collection of cogs.

“But it is a collection of cogs!” Lazlo said aloud.

Collections of cogs don’t behave the way it does, that stubborn contrarian voice responded.

“They do if they’re directed to by someone else!”

Who? Master Ibraham? You can’t seriously believe he has the skill. Or the imagination.

“Well, no, no, I suppose not,” he told himself. “On that we agree, at least.” He raised the mug to his lips. “Still, how can I know it’s alive, merely because it says it is?”

How can you know anyone is alive, merely because they say they are? You might be the only real person who exists. Everyone else could be a simulacrum, a product of your own delusions. How would you know otherwise?

“I’m not having this conversation with you.”

Of course not. You’re having this conversation with you.

“Well, then, yes, I suppose I am, aren’t I? But that’s hardly the point.”

Principle of greatest caution, then. If you treat it like it’s alive and later find it’s not, you will have wasted a bit of time, perhaps. If you treat it like it’s a thing and it’s not, you may destroy something remarkable.

“Yes, well, yes, I guess that’s a good point.”

Obviously. You thought of it.