Ask the Consul
I am married to a U.S. citizen (or a legal permanent resident) who is petitioning for my son (or daughter) for a residency visa and I will soon take my child for the interview at the Consular Section. How should I prepare?
First, follow the instructions cited in the materials sent to you for your interview. Second, you should come prepared to show the validity of your marriage, that you and your spouse truly have a life together. There's no "formula" or "specific list" of documents or evidence to bring, because every marriage is different. Many people receive very bad advice on how to prove a marriage. The most important thing to do is to tell the truth. It is recommended, but not required, that both spouses (the stepparent and the biological parent) attend the interview.
Since your child is eligible for the visa as a result of your marriage, the consul is likely to request evidence of an ongoing relationship between the stepparent and stepchild, that is between your spouse and your child. Such information depends on your particular situation and is not the same for everyone. Evidence can consist of anything that demonstrates an ongoing relationship, both between you and your spouse and your child and your spouse. Generally, applicants come well-prepared to talk about the parent-child relationship. However, the visa also depends upon the validity of your marital relationship: Your child must truly be a stepchild of your spouse.