I have had this vague memory of watching a movie about a cartoonist when I was a kid. I knew that it starred Jack Lemmon, and I can vividly remember the main character working on a comic strip with pen and ink. It was a revelation to me at the time that comic strip artists work on large pieces of bristol which is later shrunk down. Up until that time, I can remember looking at the newspaper comic strips and thinking, "Wow, how do they draw in such tiny squares." This movie set me straight.
After an internet search I learned that the name of the movie is "How to Murder Your Wife" (1965) and that it was available on our Blockbuster website. After reassuring my wife that she was safe, I ordered the movie and we watched it tonight. There are some very funny scenes and it was neat to see how the main character projected his Walter Mitty fantasies and everyday frustrations into his comic strip. However, it is full of 60's era "dames are gonna be the death of us" chauvinist humor and a very dumb resolution to a courtroom scene.
The comics that you see in the movie were actually drawn by cartoonist Mel Keefer, a cartoonist who is actually still working today. Here is one of the strips that appear in the movie.