Throughout the film, Hepburn's character, Ethel Thayer, is wobbling in a fashion that some older people do when they start to lose motor control (sometimes as a result of a degenerative disease).
Was she acting this, or was she actually affected by such a condition?
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 head, particularly during her performance in "On Golden Pond." These are classic symptoms of essential tremor that has affected the head and voice. She never appeared to have any difficulty walking. Ms. Hepburn did not have Parkinson's Disease, and despite her "disability," she was able to perform for many, many years."
Here is an additional excerpt from a New York Times Q&A where someone asked "Why did Katharine Hepburn's head shake?":
Katharine Hepburn's ailment, recently discussed on television by her niece Katharine Houghton, was not Parkinson's disease, but a progressive yet treatable neurological disorder called essential tremor. Ms. Hepburn's was referred to as familial tremor, the inherited form. Each child of a parent with the disease has a 50 percent chance of inheriting a gene that causes it. In other cases, there is no family history.
The disorder, which varies in location and severity, used to be called palsy. The International Essential Tremor Foundation, which offers information and helps search for a cure, says the disease stems from abnormal communication among areas of the brain, including the cerebellum, thalamus and brain stem. The group estimates that 10 million people in the United States and 200 million people worldwide have some form of the problem, often undiagnosed.
Many do not seek treatment because they fear Parkinson's or think tremors are part of aging. Shaking, usually starting with the hands, can begin at any age but is more common in older people; tremors starting with the head are more common in women.
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.
