IT WAS AN invitation no book reviewer could refuse. Several weeks ago, well in advance of the pre-Christmas turmoil, I and a dozen other regular reviewers of fiction for Book World received an invitation to participate in a holiday symposium on this tantalizing question: "Which character in fiction would you most like to be, and why?"

Not in the least surprisingly, none of those offered the chance to participate in this novel seminar declined to do so. For some, as you can see from the pieces below, the choice was easy; for others it was difficult, torn as they were among a number of characters who had managed to work their way into their hearts.

At times I've wanted to be somebody else. When I was an advertising writer, for instance. Or when my kids were teen-agers. Or the afternoon the psychiatrist took another puff on his pipe, looked me straight in the eye and said, "It sure would be nice to have two wives."

Right now, I'm O.K. as me. Still, put me down as volunteering to become Guy Grand, the billionaire practical joker from Terry Southern's The Magic Christian (1960).

He's the fellow who pays a smart aleck $6,000 to eat a traffic ticket, launches (with stellar endorsements) a shampoo called Downy that actually makes hair stiff and wiry, and who secretly laces the passenger list of his luxury cruise ship (The S.S. Magic Christian) with freaks, lunatics, and real gorillas. It costs him millions.

As Guy Grand today I would continue his mission of "making it hot for them." Russian cosmonauts would be bribed, through the resourceful Bulgarians, to touch down on the White House Lawn bearing jelly beans. Out in California a theme park to out-Disney Disneyland -- Peaceland, where the rides are on MXs and Cruise missiles. Bomb sights for drinking fountains.

These are just for openers. As Guy Grand says, "These are odd times . . . times that try men's souls . . . Each of us does his best."