Above nicked from Lines and Colors.
GASOLINE ALLEY cartoonist Frank King on his process:
My Skeezix is five years too old. He is nine years of age and I have to remember back for material from that source. I also must remember a lot of things which didn't happen but which might have happened. That is one of the fortunate things about human interest stuff. You are not limited to what has happened, but you must not do things which would never happen.
Many ideas are sent in, but very few of them are usable. Many come in in some form of propaganda, which have to be weeded out carefully. A few ideas come from friends' children.
Go read the entire 1926 interview at the Stripper's Guide.