Technically, there is an obligatory USCIS interview after filing Form I-751. But as a matter of practice, the interview is often waived if a couple filed joint petition as required and provided explicit evidence of a bona fide marriage. USCIS has to make sure that your marriage is not fake, fraudulent or sham concluded just to receive US citizenship.
So you can avoid the interview if together with Form I-751 you will file a complete and well-prepared evidence of a good faith marriage. For example, you may provide: birth certificates of your children born to the marriage, documents proving that you live together and own property together, photographs of you and the spouse together.
Only when the examiners find any discrepancies in your data, have some questions or doubts as to the bona fide marriage USCIS schedules an interview. The following issues may raise suspicion: the married couple is not living together; a large age difference between the spouses; a prior I-751 had been denied; the petitioner fails to sign the form; insufficient evidence of a bona fide marriage. Attendance of the interview is compulsory, otherwise you risk to lose your conditional resident’s status and to be deported.
If your were summoned to the interview do not be afraid, just prepare as much evidence concerning your good faith marriage as you can, and get ready to answer questions about your spouse and your relationship.