He smiled, I took his picture, and then he went back to his nap. The film was using the location on the canal to shoot a ferry crossing, a big raft that was moved from one side of the canal to the other by pulling a rope that was strung across the water.
The name of the movie was "My Name Is Nobody," and it was produced by an Italian film company. The movie was filmed in part in New Orleans also.
Here's the photo, which was published again in 1982, shortly after Fonda's death, to commemorate his visit to the St. Tammany community. Click on the image to see a larger version.
Here are some additional photos of the film location, the ferry going across the canal, and the shack on the other side where people were "fishing."
Fonda is on the right, pulling on the rope.