Yep, Cornflake is totally right about hospital admits--you need a codable diagnosis. But diagnoses are quite helpful. They definitely guide treatment. It's big difference between treating a trauma victim vs someone personality disordered. And actually, after one's been doing it for awhile, you do get pretty good at doing it quickly--at least getting a solid working diagnosis. Certainly one can be wrong (and modify it.) I' ve worked with some outstanding psychiatrists would really, would have someone diagnosed very quickly, just because they've heard a lot of stuff before. My supervisor in grad school nailed my MMPI code!
But the client is relevant here too in the process. I've worked with a lot of mandated clients, and you can see a lot of pretty but totally irrelevant country before you get close to what might be the main destination (one might have to work harder.) And that's diagnostic. OTOH, you can have clients who present with, "So I've had recurring depression for years but over the past two weeks it's gotten a lot worse. I've gained weight, I sleep all the time and I've starting wondering if the world is better off with out me. I used to take a tricyclic antidepressant, but it didn't work that well, so I was wondering about maybe trying an SSRI" ie, clients who have a lot of insight and make the job a lot easier - oh Im a recovering alcoholic struggling with my dad's drinking/I was abused as a kid and thought I was doing pretty well, but now that my daughter is at the same age, I m really struggling/I need to talk about whether or not to leave my wife, etc
And after you've been doing it awhile, you can also pick up on less than truthful reports (which are also diagnostic-why would someone minimize or exaggerate symptoms?)
for the purposes of the story though, I agree with getting a copy of an intake form, which will show the types of info generally sought. And yes, if a client came in asking, "So do you think I'm crazy?" a provider is likely to ask them what makes them wonder about that, ie what brings them in, what's the issue. One hopes that what follows will clarify that - I sometimes mistake my wife for pigeon, the morning DJ talks directly to me, I want to marry outside my religion, I have fish in my underwear, I want a divorce - whatever.
And a doc will want a history (how long?) why the person has tried, any medications (this can be a big clue, if the person presents a list of antipsychotics, or an ADHD med, or an anxiolytic or something, that would be a tip off about what someone
else thought anyway)
one is also listening for how a client tells their story ("fucking cunt PO sent me here because she's too stupid to see I've already done craptons of treatment," vs "well, I got another DUI and my PO thought I better get some help. I agree. I need to stop drinking!" ') because as was pointed out, one is not only just listing a code, but also impressions (thought, language, insight, judgment and so forth)
Again, for your story, it would not be unrealistic for a client to make an appointment with a private practitioner, come in and ask if s/he thought he was crazy, and then be asked "why would you think you are?" And he would just tell the shrink his symptoms (the nightmares.) You don't need him to have some crazy ass complex diagnosis, right? IF it's not involving insurance, he doesn't need a specific diagnosis (although IRL one still has to have a case conceptualization about what's up with a client, but in your story, you probably don't need that- he could just talk about his nightmares) The shrink would certainly be interested (is there a history of trauma? Is the person on medication/have a drinking/drug problem? Family history of psychosis? And so on) Your story is on the guy with the nightmares, right? Not the shrink.
good luck!