Sending out lucky thoughts! I know a few people to get their green card and they have all said it wasn’t too bad.
I think your comment about it being fairly obvious when people are genuine is pretty spot-on.
Somewhat related, I had a conversation with a friend whose friend was an immigration person - they conducted the interviews for a while, then moved on to something else. Of course, this is just hearsay, but apparently they do watch people fairly closely in the waiting room while you’re waiting for the interview to read what their interactions and body language indicate about the nature of their relationship.
He said that two people who send up red flags are people who are all over each other and those people who do everything they can not to touch each other. The people who are all over each other garner additional scrutiny because they’re trying to hard to prove they’re in love, while the people who try hard not to touch each other are clearly there for financial purposes. The people who don’t garner as much attention are those who act like a normal couple - you know, knees bumping occasionally, chatting, companionable silences, clearly enjoying each other’s time, - just basically relaxed and comfortable with themselves and each other.
Ours was pretty easy. We started fighting over when we first started dating & apparently, that was all they needed to hear. Approved.
Good thing too, because she never learned a thing about my personal preferences, like what soap I used (unscented because I’m sensitive to perfumes)!
My memory is hazy. As I think about it now, the reference letters may have been only needed later on when you file form I-751, the petition to remove the “conditional” status of your green card. And you won’t particularly need to bring them with you; I think they end up getting mailed in along with I-751.
Best advice is to not take anyone’s word for it here: follow all the instructions given to you by the good folks at USCIS as carefully and completely as you can. Where judgment is called for (e.g. “bring evidence of your relationship”), you probably want to deliberately err on the side of caution (e.g. bring abundant evidence of your relationship).
Once you get your conditional permanent resident status, check carefully on the time requirements for when you are supposed to file form I-751. IIRC there’s a window for filing that starts 21 months after you receive conditional permanent resident status, and ends three months later. Mark you calendar so you don’t miss that window!!!
Me: White
Wife: Asian.
Process: Painless. We had garbage for documentation too. No shared investments or mortgage, just a bank account with both our names on it and some random personal junk. I think the only question they even asked was “Tell me something about your partner that annoys you.” and if you can’t answer that, well, then you’re not really married. 
British guy here, previously married to an American (both white), went through the process 15 years or so ago.
It’s easy. Our guy only cared about the paperwork we brought - especially the joint lease and joint bank account. He hardly asked us anything about our relationship, and I am 99% certain they never separated us at all.
I wouldn’t worry - though I know you will as I did. But get through it, and enjoy the champagne after.
Another piece of anecdotal evidence: I went through this process three years ago (and one year ago to get the conditions removed). The interview was easy, in fact I don’t remember being asked any relationship related questions. I too believe it will be quick and painless unless they are specifically suspicious of you, and I assume that if the officer is experienced they are a pretty good judge of this.
To get the conditions removed was even easier - I thought an interview was required but it was approved without it.
My wife and I went through this a year ago. (My wife is a US citizen, I’m from Japan and was applying for a green card through marriage.)
Our interview was a bit more thorough than what others here have described, possibly because our marriage is somewhat unconventional (age difference, no children, etc). They first asked some personal questions, e.g. what we do, where we met. They then gave us each a set of questions about each other and asked us to write down the answers without communicating (same room but facing away from each other). I don’t remember all the questions, but I think some were: “What medication does he/she take?”, “When/where did you last travel together?”, and “Name 5 of his/her best friends”.
As for documents, we didn’t have reference letters at all, as my lawyer thought it was unnecessary. Besides the required documents (ID, social security cards, marriage certificate, etc) I recall we had copies of joint tax returns, account info on our joint bank account, proof that we’re covered under the same health insurance, and a few of wedding photos and family/travel photos. I can’t remember what else, maybe a couple pieces of mail with same address under both names.
The approval notice arrived only a couple of weeks later.
Ok, my wife is from Iran and it was no big deal at all. Iran.
Don’t worry.
She’s not Iranian – she’s Persian. She’s from Persia.
It sounds like you think you’re being funny, but I’m afraid I don’t quite get it.
In the vast majority of cases I’ve known about personally (including a number of friends, not just clients), there were very few questions. Provided you look and comport yourselves like a couple, and have some kind of documentation of oyur relationship and some kind of joint finances (even if bills aren’t sent to you jointly, for example, it can be helpful to submit things that were only addressed to one of you as long as they show you are both living at the same address), you should be fine.
The only case in living memory where there was any kind of an issue in which the couple didn’t have any red flags (large age difference, huge differences in socioeconomic background, not speaking a language in common, that kind of thing) was a recent client who had the bad luck of drawing a really hardass officer from the same ethnic background as hers, who had a rep for not wanting to seem like she was being too easy on “her” people. But really, the overwhelming odds are that you’re going to be fine. Just relax!
Thanks for all the stories and advice! I’ve got a big stack of paperwork all organized in a folder, I printed out a bunch of pictures, and I’m going to make sure we get there early with plenty of time to spare. We’ve got a mortgage and car loan under both our names, a joint bank account, and engagement and wedding photos, not to mention photos and emails and flight confirmations from the past 5 years, so I doubt they’re going to think we’re in a sham marriage. My biggest concern is that we’ve forgotten the one document they’ll ask for, and everything will be delayed.
I’ll also try to remember to breathe. 
Good luck! Based on this thread, I don’t think you’ll need it, but take it anyways.
Worse: she’s a scientist!
I knew someone who had a similar interview, but for herself only: she was Costa Rican, entering the US as the fiancee of a US citizen, the interview was for the Visa. He was a graduate student, she the older sister of another graduate student; they’d met when she was visiting her brother and hit it off, and here she was a few months later, entering the country to marry a several-years younger man she’d seen in person once before. He was wrapping up a PhD in Chemistry, she had a Bachelor’s and Master’s in the same field.
At one point, the interviewer asked her to draw the formula of acetone. She spit out “doubting that I love him, I understand - my family are the first ones scratching their heads and questioning our sanity. But do you NOT dare doubt my professional qualifications, damnit!”
She did get in, by the way
It’s not uncommon for Iranians to refer to themselves as Persians to distance themselves from the Islamic regime. Almost half of Iranians are ethnically Persian.
It went well!
The woman who did our interview was young and friendly and in a good mood, and it was all much easier than I thought it would be. A few questions about our families, where we met, that sort of thing. As she went through the photos she asked a few questions about them, and commented on how awesome it was that we’d traveled so much. She also exclaimed over the quality of our photo books and asked where we got them made (Snapfish), and asked where we got married because the place was so pretty. She also complimented me on the bridesmaids dresses!
At the end of the interview, she said that as far as she’s concerned everything is in order and they shouldn’t need anything else to approve me, but someone else needs to look over the file first before anything can be done. So I have to wait again. If anything is missing, I’ll get a letter within a couple of days asking me to bring it in. If everything is fine, I get a letter in a few weeks saying my green card will be on its way soon.
Huge sigh of relief to have that out of the way.