Paul does a Bogart imitation, while at the speakeasy. The scene takes place in the mid to late 1920s, either during silent movie era (He would not have even known what Bogart's voice sounded like), or early talkies. Bogart was an unknown in movies until the 1930s.
When Norman returns to Missoula after college at Dartmouth & meets Jessie(in approximately 1923), she speaks with a stereotypical New York accent. Tough to copy since sound films didn't appear for at least five more years.
At the Labor Day picnic, Reverend Maclean is playing horseshoes when Paul arrives. Paul addresses his father by saying "Hello, Father." Although you can see Reverend Maclean say "Paul", the dubbed voice says "Son".