Saw this in one of the comments for
Red Velvet 'The ReVe Festival' Day 1 Album First Listen. Unfortunately, I don't have the source for this at the moment, but I think it's worth mentioning anyway:
I saw an interview with Wendy where she said that SM has their harmonies well-mapped out before they even hit the studio. But she said her favorite part of the recording process is AFTER all the main vocal tracks are laid.
That is when they let her and Seulgi spend the day in the studio improvising all those wonderful ad-libs RV albums are chock-full of. She said that is when the songs really come alive for her.