Yes we did the interview. Before you file check this website to see which center has the shortest processing time:
https://egov.uscis.gov/processing-times/
Make sure before you file you can show you have LEGAL custody of the child. When I was finally contacted by USCIS for the interview, I had to choose an interview date within 90 days. If I didnt already have a B1/B2 visa for my son this would have been a problem so make sure you can just pick up and travel when they contact you. I was also sent a request to show the child has been living with me since I filed. I provided them with my drivers license and electric bill showing the same home address listed on his school papers.
I brought with me all the original papers of the PDF files I sent them. Birth certificate, adoption papers, my passport, 2x2 passport fotos. We are also told to prove he entered the country legally for the interview so I had a photo copy of his entry stamp and I downloaded his I94 showing when he entered. https://i94.cbp.dhs.gov/I94/#/home
For the interview, she asked my son what his full name is and his date of birth. Then she asked him what my name is. Then she asked how tall he was. She asked us to wait outside and 30 minutes later brought us back in and had me and him sign his citizenship certificate. That was it. I only have to do this 2 more times for his sisters. :)
Good luck