On November 30, 2018, USCIS published new guidance on interviews at the I-751 stage (you can read more about what the I-751 is here). This policy went into effect on December 10, 2018, and applies to all I-751s received on or after that date. The significant change that is occurring is that if the principal petitioner was not interviewed at the stage of when the beneficiary first received the two-year conditional residency, then there will be an interview at the I-751 stage with USCIS. The type of situation this would apply to is where the principal petitioner petitioned for a spouse, and the spouse goes through the U.S. consulate abroad to receive an immigrant visa to enter the U.S. as a lawful permanent resident (I talked about consular processing versus adjustment of status in this blog post).
When the foreign-born spouse goes through the U.S. consulate to immigrate to the U.S., only that spouse is interviewed by the officer. When a couple gets married in the U.S. and the couple remains in the U.S., the couple will be interviewed together at a local USCIS office.
In summary, for cases where the couple goes through USCIS and both are interviewed by a USCIS officer, the interview at the I-751 stage can be waived if USCIS believes that there is sufficient evidence to prove that the marriage is genuine, there is no indication of fraud or misrepresentation in the I-751 form or supporting documentation, and there are no complex facts or issues that require an interview to resolve these questions or concerns. Otherwise, for couples who went the consular process route and only the foreign-born spouse was interviewed, the couple will be interviewed at the I-751 stage. This is a departure from the previous policy where USCIS could waive the interview in these scenarios if it is determined that there is enough evidence in the record to establish the bona fides of the marriage, and there are no complex facts or issues.
See the USCIS policy memorandum here.