But, for me, it was quick and painless. I will describe my own experience, so that others will have a better idea what to expect. Keep in mind, each case is different, and your experience may vary significantly. I generally have an easy time with USCIS, so maybe I just got off easy.
First off the documents. What they DID need to see:
- Green card (for naturalization eligibility).
- Washington State photo ID (to establish residence/jurisdiction).
- Passports from all my claimed nationalities.
- Originals of any documents I submitted copies of with my N-400. This includes documentation on spouse's status, children, evidence of bona-fide marriage, of current address, etc.
- Tax returns or transcript.
- Photos. In fact, they had my originals on-hand (which I had to sign), plus digital copies.
- ANY copies of anything. I did not leave a single piece of paper with them.
The first step was a review of my application (the N-400). This part is no joke! They will go through every page, testing your knowledge, the accuracy, and filling in details. The hardest part was when I was expected to recite my anniversary, my wife's birthday, and my daughter's birthday in short succession (they're close together and easy to mix up).
One specific step that worries some people are the list of trips outside the country. They certainly did ask me about these, but my perception was that they weren't looking to trip me up on technicalities at the interview stage. They asked me to briefly talk about a couple of my trips (notably the ones not to Canada), but didn't really have any clarifying questions. Notably, they never asked me about specific date ranges or to account for my totals for number of days outside the country.
English was trivial. Read "Who can vote?". Told to write the answer "Citizens can vote." Six words. I was disappointed that they do not automatically make every Canadian write a sentence with the word 'color' in it.
Civics and History test - well it's exactly what they tell you it is. You already know the possible questions and answers. Study and it will be fine. 6/6 for me.
After the interview is finished, they give you a form N-652, documenting whether you passed/failed and the disposition of your application. For me, they recommended approval on the spot, which from looking around the lobby crowd seemed to be a common outcome. What I didn't expect is that they handed me my notice for the naturalization ceremony right there at the interview. The bigger surprise - they scheduled my naturalization ceremony for THE SAME DAY! My interview was at 8:20am, my ceremony at 11:40am. This did not seem to be a coincidence, as I recognized several people at the oath ceremony (total ceremony was about 60 people) who had been in the testing waiting area that morning.
So yes, I left to Tukwika expecting a morning at the central bureaucracy, and left a citizen.