You will or you’ll, there is or there’s?

What would the character say?

Most people often use contractions; at least the ones that I hear do.

There's the matter of showing ordinary informal speech, yes, but there's also the matter of emphasis.

The mother who says, "You will eat your spinach." is being slightly more emphatic than the mother who says "You'll eat your spinach."

And then there's the matter of rhythm and flow--subtile elements you may want to consider as well.

Character and context. Character and context.

If you want to make a character sound precise, educated or mechanical than you make sure they use few or no contractions. Spock or Data in Star Trek. Her Majesty the Queen. A megalomaniac villain: "Ah, Mr Bond, I have been expecting you."

By contrast, someone using lots of contractions sounds more down to earth, blue collar, friendly, informal.

Then there are times when you want to stress a particular word to make a point. Here is the famous speech from the movie "Taken":

I don't know who you are. I don't know what you want. If you are looking for ransom, I can tell you I don't have money. But what I do have are a very particular set of skills, skills I have acquired over a very long career. Skills that make me a nightmare for people like you. If you let my daughter go now, that'll be the end of it. I will not look for you, I will not pursue you. But if you don't, I will look for you, I will find you, and I will kill you.

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For the most part, this character uses contractions. There are three "don't"s in the first three sentences. He's an ordinary guy, talking one to one with the person who has kidnapped his daughter. Let my daughter go and "that'll" be the end of it.

But he switches to avoiding contractions when he is making his threats and describing his skills: "I will not look for you" instead of "I won't look for you."

Then we have the threat: "... I will look for you, I will find you, and I will kill you." The emphasis is on the word "will" - also repeated three times.

This is far more effective and calculating than "I'll look for you, I'll find you and I'll kill you."

In other words, repeated use of contractions for pleasantry and repeatedly spelling words out in full for threats. Cold, calculating, professional - just the image that the character wants to give.

https://www.youtube.com/watch?v=fUlGa96L7_Q

msd said:

I’m struggling with the problem of knowing if I should put an apostrophe or not in a dialog. I'm showing you an example below of what I mean. At this time I choose what sounds right to my ear. However I’m sure there’s a rule I should use instead.

“You will notice that there is a large red border around the doors.”

“You'll notice that there’s a large red border around the doors.”

Is there a rule for this kind of situation? To me the second line sounds right.

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You write it the way that particular character would say it, but get rid of "that". It isn't needed in either sentence.

You'll find there's lots of good advice above.

In dialog, it really just comes down to how the person speaking would say it. Personality, the degree of emphasis he/she is placing on his/her words, and of course, the flow of the sentence itself, will influence this. Try reading it aloud to see if it's something your character is likely to spit out in that situation. Also, how does it affect the mood or tone of what your character is saying? Does it mesh with your intent?

Think of the difference between someone saying, "We will go to my mother's this weekend," versus someone saying, "We'll go to my mother's this weekend."

To me, the former reads more like a mandate, or or maybe something said between gritted teeth, while the latter is more a simple statement of fact or intent, made in passing.

As for associated narrative, the same sorts of rules apply, with less emphasis (perhaps) on ease of spitting the words out. Is the pov character (or omni narrator if you're using that narrative viewpoint) emphasizing the words? Are they someone who tends to think more emphatically or formally than the way they usually speak? Or are you shooting for a more in-voice or breezy tone in your narrative?

Jamesaritchie said:

You write it the way that particular character would say it, but get rid of "that". It isn't needed in either sentence.

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It's absolutely needed if the character would speak that way. Some people do. There are people who use "that" as a subordinating conjunction in their every day speech even though it isn't necessary for comprehensibility.
It's entirely possible that MSD's character is one of those people.

msd said:

I’m struggling with the problem of knowing if I should put an apostrophe or not in a dialog. I'm showing you an example below of what I mean. At this time I choose what sounds right to my ear. However I’m sure there’s a rule I should use instead.

“You will notice that there is a large red border around the doors.”

“You'll notice that there’s a large red border around the doors.”

Is there a rule for this kind of situation? To me the second line sounds right.

Click to expand...

Most people speak in contractions. The second example is fine, and preferable.

Agree with above and the suggestion that 'that' usually adds nothing unless needed for particular emphasis.

Read dialogue aloud and ask yourself if what you have written is how your characters would say whatever it is in whatever the situation is.

There's a big difference between writing fiction and formal writing for a class or journal.

The rules about contractions that apply to formal writing don't apply for fiction, and they're even more relaxed for dialogue in fiction.

People use contractions, slang, sentence fragments, incorrect verb tenses, etc.

I say, write what the person would say, the way they would say it. If the character is one who would say "I ain't no doctor," then that's what you write. You don't correct his grammar to "I am not a doctor."