Under game settings, you can set " whisper rolls to gm " to "always whisper rolls". This will update the default for new characters and then you can later apply the default to all characters in the game if you choose. To individually change an NPC character, you first have to open the character for modification. Move your cursor to the top left of the "Character Sheet" tab and you will see the cog appear; click it. Scroll down to "Options", where you will find the option to change the setting for " Whisper Rolls To Gm".