There was a time, before CGI ruled the world of movies, before every hit movie was a sequel, a remake, a sequel of a remake, or a remake of a sequel, that everyone in Hollywood agreed that The Lord of the Rings could NOT be made into a motion picture. This was well documented. Apart from it being too long, its vastness, scope, and scale were considered insurmountable--not to mention the need for an ensemble cast, which Hollywood had given up on for any movie, at the time. Easier to hire one big star and market from that. Ralph Bakshi had attempted The Lord of the Rings as an animated film--he was very loyal to the book, much more loyal than Peter Jackson (to a fault, many have since said). He did half the "trilogy" (Tolkien said it was one book, till the day he died) in its first installment. And it failed miserably. The other "half" of Bakshi's Lord of the Rings never got made.
Anyway, during this time, between the time the Rankin Bass cartoons of The Hobbit and The Return of the King were made (circa 1977) leading up until the 1999 announcement of the Jackson Rings trilogy, it was common practice among the nerds to "cast" The Lord of the Rings--for fun, as though we could really do it.
Within my group of friends, there were two choices we agreed upon, without regard to accent or nationality. It was about character attitude, about a vibe they would bring to the role.
One was David Bowie as Elrond. Don't buy that one? Watch Labyrinth again.
The other was Robin Williams as Tom Bombadil.
I see my fellow nerds out there smiling.
You know it would have been perfect.