Had my interview at Montreal today and wanted to share my experience. Came 5 min early and had to wait for ~20 min outside since there was a line to get in.
As others said it started from a security check: small bags were allowed, no backpacks. There were plenty of lockers available to leave your phone there. It's not that tiny actually - fits 2 wallets and 2 phones easily. After that we were given a yellow card with the list of documents to keep ready and a ticket with the number in line. Had to wait for an hour to get called for documents check.
To those asking about the employment letter - mine did not have a wet ink signature or stamp and it was all good. My employer sent it to me by email so I just printed it out and there were no questions asked.
Someone here mentioned that they do ask for a passport with US visa if you had any. I had US tourist visa in an old expired passport and was asked to show it.
After that waited for another ~30 min to get called for fingerprints and 40 more min for the interview. Interview took about 15-20 min total and they did not ask for any additional documents in my case. I'm EB-3 and was asked to describe my job responsibilities, where I am planning to work in US (my employer has several locations in US). Also questions like if I've been to US before, how long we've been married, if either of us had any problems with visas in the past, etc.
Was approved by the officer and told to wait for 2 weeks to get our passports back.
The whole thing took 2,5 hours.