Your spouse "Francis" has gone to the embassy for his interview. Everyone believes that the marriage is bona fide but the problem is that he was in the US illegally for several months/years and they want a 601 Waiver to be filed showing extreme hardship to you, the USC if they decide to keep Francis at home.
You will want to think of EVERY way that he contributes to your financial and emotional well-being. Do you have photos documenting and painting a picture of a life together? Do you have kids together or that depend on him for their emotional and financial well-being? Do you attend a church together?
Once you have organised your thoughts, write a letter. But don't write a cold factual letter, now is the time to pour your heart out- talking about how important he is to you as a person. There is a difference between saying- "He is my husband and I miss him" and "I do not know what I will do without him, he is my best friend and companion. Every day that I wake up thinking he isn't coming back makes it difficult for me to get out of bed. The kids cry and cry and I don't know how to tell them it will be ok- but that we must be strong and to have faith". The consular officers receive many waiver requests every day and yours needs to hit them hard.
Have the kids write as well- have them draw pictures of what they feel the separation is like. Assemble copies of the photos in to support your story and write in captions- "This is the yard when we bought the house- this is how it looks after all the work we put into it together". "This is the pen where we keep the bunnies that we raise."
The priest or minister at the church can write a letter too- discussing how your family is an important part of the congregation and that you are suffering as is the church as a whole. If you have been depressed due to the separation- can you have the minister and psychiatrist attest to the emotional and physical side effects you are suffering from the enforced separation?
Be prepared to discuss the financial aspect as well. If you have had to sell a car because you can't keep up with the car payments or you had to move because your husband isn't here any longer. Talk about why you CAN't go and live in his country. If it is because there is a lack of medical care for a condition you suffer or an inability to work there- bring it up. You will want to include in your filing EVERY thing that you can think of to help bring him back.