You need to know at least:
- full name of your spouse
- spouse's DOB
- how old is your spouse
- spouse's employer
- spouse's occupation
- date of marriage
- when and where you met
- names of spouse's close relatives: siblings, parents etc
- address where you live
- whether spouse was married before
- spouse's level of education
- where the spouse was born
In addition, many personal questions. The list will be too long to include, but some examples:
- on which side of the bed does your spouse usually sleep
- what time does your spouse typically leave to work
- what is spouse's favorite color
- what all languages your spouse can speak
- what car does your spouse drive
- what hobbies does your spouse have
- what did your spouse eat this morning for breakfast?
I'm surprised people don't talk about it. One of the things my lawyer did was running a couple of quick mock interviews with me and my US citizen spouse to ensure we answer questions correctly, consisely and with confidence.