In some ways you need to wear many hats. You need to be an actor.

You need to pay attention to the world.

For me personally, I envision scenes in my head as if I am watching a movie. But I don't draw from thin air. It is just the culmination of everything I have ever seen, experienced, every movie I watched. Every documentary.

The question shouldn't be, what do you want them to say. It should be, What would they say.

It may sound odd but you need to take on the role of the character, their traits, background, and let the behavior and dialogue emerge wherever it takes you. You can always edit and guide it later or it will often surprise you and give you natural directions on where the scene can go.

The trick is to say the dialogue out loud to yourself while you work or when checking it.

You've been hearing people talk your whole life- you know what natural conversation sounds like. It's easier a lot of times to pick it up.

Take turns pretending to be each character, doing the best impression you can

Oh man. That's a hard one.

I think the only thing I can offer is to practice.

When I was just starting out I would do a writing exercise where I would have a single scene and then rewrite in different styles and themes based on a prompt deck.

Ie. Same characters and goals but first time through it's a horror. Then a cozy conversation over tea. Then an interrogation and one is a cop. 

I did this over and over with new scenes until I got to the point where I festive I could pick up a prompt and write it well consistently. It took years of practice.

Also take some writing classes that do prompt work. Then you get to see what others do with the exact same inputs.

You need solid characterization to get quality dialogue. If you don't understand your characters, it's impossible to write smooth, natural conversations.

Once it's written, read it aloud. If it sounds robotic, stilted, awkward, etc, it probably is

Uh, you can write evolutions of the same people and world for 35 years. That’s what I do. In all seriousness, though, put yourself in each character’s shoes, one at a time.

-What do you know about the character?

-When faced with a scenario, what does the character do? What you know about the character should help you answer this question. If it doesn’t, you don’t know enough about the character.

-If you can’t come up with plausible in-world scenarios, find or create a list of writing prompts. Go to a randomizer/dice roll prompt (you can find them using Google). Set the range to the length of your list. Randomize/roll. Use the prompt associated with whatever number comes up.

It’s a start.

You need to see the scenes in front of you

You have several questions here, which aren't as tied together as you think and that might be why you feel so discouraged.

I don't become automatically immersed. I've pavloved myself with regular habits and music to get into the mood when I sit down to write, but even my flow state doesn't look like me seeing the scene perfectly and typing it out correctly on the first try. That happens very rarely and only if I've already daydreamed the whole scene while doing chores. Usually I sketch out the scene, writing something like:

A: lists out the reasons why this is a bad idea. (walking up and down the room)

B: sarcastic deflection (tries to stop A, fails)

And so on. Maybe with some very bad, cringy dialogue sprinkled in. Once I know WHAT I want to write, I go back and I can then focus on HOW these characters would say these things. How that happens comes from a lot of experience acting and observing others and just practice. No shame is acting out irl.

And I stay motivated because well... it's fun? It's fun when I come up with these things, I giggle to myself when I have a good line or become surprised when I have a good twist, and there is the deep, rewarding satisfaction of seeing your story grow and come together and then there is the joy of getting feedback. Few things bring me as much pride as when I finish a project. I daydream about the story while at work and coming home to write it while blasting my fav songs is literally my reward for surviving the day. Doing a little bit every day or every few days also helps to keep you "in" the story, I find.

Stephen King says he puts himself into a trance when writing. Not literally, but as close as he can get. He does this by repeating the exact same routine every single day. He gets up, has breakfast with his wife. Then he goes to his study where everything always is the same—same stuff on his desk in the same place. He has water and juice for hydration. He listens to some music to get him into the story, then turns it off. He always leaves off with something unfinished, so he starts a little back, reads, and when he gets up to the unfinished part, he just continues writing from there. He writes for 3-4 hours without stopping.

That is one way that one author gets fully immersed in his story all over again every day.

Another vote for reading aloud.

I’ll answer your questions in the order they appear.

• I write dialogue without recording first by having done so so often before. That is, practice. Now your method is fine! It actually sounds interesting, and I’ve done it in a different form before without recording. Just acting it out to myself or what not.

• I immerse myself in the story by being invested in my story. I put on music I attribute to it or the themes. I remind myself of who these characters are, their motivations, their goals. I put myself into their shoes. This is also how I get motivation.

I dunno. I've always had a very active imagination, so most of the time I feel like a courtroom stenographer just writing down what I see in my head.

A lot of writers struggle with this, so you are not broken. For dialogue, it helps me to stop thinking about “good lines” and instead focus on what each character wants in that moment. If you know their goal, the words usually follow more naturally. I also like writing rough, messy dialogue first and letting it sound bad, then fixing it later when I am less inside my own head. Immersion is weirdly unreliable, so I try to build habits that make it more likely, same music, same time of day, same notebook or doc. Motivation tends to show up after I start, not before. Acting scenes out is not a failure either, it just means you found a tool that works.

Have you played dnd? Or perhaps played any rpg game? Because that's what I do.

You roleplay, don't think of it as acting, you are not an actor in that moment, you are the character.

You. Not the character you think they should be. You are the character. I cannot stress this enough for this to work.

You don't have to act out your scenes or stories, you get inside their heads like someone else already said, "you have many hats to wear for your characters".

So far how have you characterized your characters?

How they think, behave, speak, act. You need to internalize that for you to write dialogue that feels true to their character.

Not necessarily good(that's kind of subjective). But true to their character.

How can you exspect the reader to be immersed in a story that you are not immersed in?

It sounds like you have not found a topic to write about yet and that makes me wonder why you want to write a story?

First you smoke little weed, then proceed with a bottle of Jack, have some cocain, and then drop an acid

My advice is to just write. Write crappy dialogue, and just keep going. And going. Soon you will know your characters better, and you'll find the dialogue getting better and coming more naturally. Then you can go back and improve the dialogue that was crap.

I don't entirely know how I do it, except to say that my characters live in my head. I get to know them by seeing them in action, and the better I know them, the more naturally their dialogue (and actions) flow.

I write what I'd like to say or hear than adjust based on the character setting emotion etc

MPD.