One small thing I think people intuit without realizing is that part of the "He would not say that" is that, beyond the big-picture concerns (where you really mean, "he would not be expressing that sentiment" or "he would not be saying that to that person's face" or "he would not be saying that thing out loud"), there's the close-up concern of vocabulary used.
Sometimes where writing, particularly dialogue, can feel funky is the problem of voice, of that just doesn't sound like him, which can come down to individual words used. What's really interesting is this sense can ping even for characters you don't know at all, NPCs and background characters, not just the big main canon favorites that everyone knows intimately.
For example, I was writing a fic recently where I had typed out a character saying
"He was lucky he wasn't more seriously hurt."
And immediately had to backtrack because the word lucky felt wrong. I knew exactly what needed to go there instead without really thinking about it, but let's break it down a minute first.
Okay, so imagine you're me and lucky feels off, so what do you do? You turn to the thesaurus. This is what you get:
[alt text added to image; should pull through]
These aren't... wrong. (Well, some of them are.) Most of them are synonyms of lucky in various contexts, but they're not one-to-one by any means. So first you have to know that, in this spoken context of describing a person who avoided a potential negative outcome, only some of these will work, because it needs to be an adjective that has to address a moment of good luck (as opposed to a pattern or a lifetime) and avoiding that potential negative outcome by chance. Most of the time, you can sort these out by saying them out loud in your chosen sentence.
"He was lucky he wasn't more seriously hurt"
â
Original sentence, construction works.
"He was serendipitous he wasn't more seriously hurt"?
đŤ No. That doesn't make sense at all.
"He was blessed he wasn't more seriously hurt."
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Yeah, that works, in a vacuum, too.
Of the above, in the sentence of dialogue I created, the following work:
Only three. (If I changed the sentence structure to "It was ___ he wasn't more seriously hurt," I could try out a few more, maybe, like fortuitous and providential, but I'm not going to.)
Even with only three options, especially in dialogue, you have to be able to parse out what kind of person would use which. That was why lucky pinged as off to me, even though it works perfectly well in this context in a vacuum.
In my scenario, the person speaking was a highly educated, upper socioeconomic middle-aged man of authority for whom American English is a first language speaking to another man with whom he has only a professional relationship, a reason to worry about his standing within the state of said relationship, and a vested interest in maintaining a healthy level of respect and trust.
That is not a man who is going to say lucky.
I knew immediately and instinctively that he would say
"He was fortunate that he wasn't more seriously hurt."
He wouldn't say blessed unless I wanted to imply something about his religious and/or spiritual background and beliefs, which I did not. Lucky has a more common feel to it, a little more casual, and just wouldn't be the word of use for this kind of character in this situation. Reaching for the three-syllable word instead of the two, the one that echoes with a tiny bit more pomp.
You'll notice, too, that a that appeared as well, because a man like the character I described would be more particular about the formalities of grammar, even in cases where his meaning is clear without them.
A different character, someone of a lower socioeconomic status and/or in a much more casual situation might even say
"He got lucky he wasn't more seriously hurt."
Do you see how those four ways of saying the exact same thing sound and feel different?
"He was lucky he wasn't more seriously hurt."
"He was fortunate that he wasn't more seriously hurt."
"He was blessed he wasn't more seriously hurt."
"He got lucky he wasn't more seriously hurt."
The exact same sentiment, just tweaked to match the speaker.
The more you start to notice vibes like this, the more nuanced and "right" feeling your writing will be. And the more you notice and start to pick apart these choices while writing, the better you'll be at it, because you'll be able to articulate the whys and why-nots and can figure out where you went wrong (and how to go right instead.)