How do you plan your dialogue/scenes scripts?

hi new renpy user here and I am starting to write my first "dummy"vn to try out and test the waters.

while I was writing the dialogues and scenes on the program I was thinking " do people really write out the dialogues on the spot or they write it and copy the dialogue off a Google doc/Word file etc...?"

Experienced VN writers, how do you plan your dialogues, descriptions of scenes, what background to use in this determined scene or what expression to do in that moment for that character, how many characters should appear on screen?