The interior walls are likely plank walls that are plastered on both sides. The plaster was the drywall of its time. They tend to be crooked and therefore quite difficult to cover with flat materials. Adding new boards on top of the existing walls also means that all baseboards and door and window trims must be removed. This involves a lot of unnecessary work. Instead: Remove all wallpaper layers until you reach the plaster (which is usually patented) and put up new wallpaper. Scrape and sand the baseboards, door, and window trims to make them look nice. If there are many layers of paint, use paint remover.