The moving scene where Grandpa (Wilford Brimley) fishes with his grandson was apparently totally improvised after Brimley suggested to the Ron Howard that his character talk freely to the boy rather than the originally scripted scene with fixed dialogue.
Cocoon sprang from an unpublished novel by a relatively unknown writer named David Saperstein. Lili Fini Zanuck, wife of Hollywood legend producer Richard D. Zanuck saw something in the story and paid $2,500 for it, developing it into the $17 million project that would be her filmmaking debut.
During filming in Florida, in the early hours of Oct. 15, 1984 with Steve Guttenberg in the car, Brian Dennehy drove away from a Bennigan's on 66th Street N, made an illegal U-turn in front of a patrol car and was pulled over.
The officers report stated the actor "smelled strongly of alcohol, staggered as he walked and did poor on balance tests."
Since Dennehy was from out of state, he was only held overnight in the St. Petersburg city jail.
During production, the Florida Coast Guard was informed of suspicious nighttime activity on the gulf. Callers feared a possible Russian submarine, but it was only the crew filming on Jack/Guttenberg's charter boat.
The Florida Teamsters union protested the decision to fill jobs with mostly out-of-state workers. Days of negotiations and rumored intimidation by Teamsters resulted in Cocoon's transportation director returning to California, replaced by a local union member.
In the scene towards the end when Bernie comes to the boat to say goodbye, Ron Howard filmed another version where he went with them, but Ron ultimately decided during editing that him staying behind was the most dramatic and poignant version.
The very first scene that was filmed didn't make the final cut. On Aug. 20, 1984, spectators gathered near Williams Park in downtown St. Petersburg where a stuntman rammed Ben/Brimley's white Cadillac into a sports car's rear bumper.
This presumably caused the DMV eye test, which he fails in a later scene.