First of all, product placement didn’t exist in the 30s and 40s. Probably the first film to use it was the Marx Brothers’ Love Happy, and that was only because the producer had run short on cash and came up with the idea of offering what would now be product placement. It didn’t become standard practice then; What Ever Happened to Baby Jane? in 1960 was still using generic cereal boxes with no brand names. It certainly didn’t become big business until after E.T.
So the smoking shown prior to 1971 (when smoking was banned on TV) was reflecting the characters and society as a whole (I’m not counting TV shows of the 50s, which are different).
As far as cigarette companies paying to show people smoke, I have never heard of anyone coming up with a single documented instance of a cigarette company paying anyone to smoke since 1989, when the FCC ruled it illegal (because movies play on TV, where cigarette advertising is banned).
Actors and directors show people smoking for dramatic reasons: it helps define a character, and it also gives the actor something to do with their hands. It makes the character more interesting.
The myth that tobacco is paying for placement is all over the place, but where are the facts to support it? And I don’t mean “Well, they’re smoking on screen, so that means they were paid.” It’s quite naive to believe that proves anything.
Evidence means: testimony or documentation that indicates an actual cash payment.