Katharine Hepburn's tremors in On Golden Pond were real. She developed essential tremors, a disease that can affect the extremities by causing them to involuntarily shake in varying degrees.
Per a website specializing in health-related information for older adults:
"...her tremulous voice and the persistent involuntary movement of her
Hepburn developed what's known as Essential Tremors http://en.wikipedia.org/wiki/Essential_tremor. It's a neurological disorder that often causes tremors of the hands and arms but can also cause tremors of the head and neck. It is thought to be passed genetically.