BACK IN the days of the Mississippi riverboats the gamblers, according to legend, won and lost plantations with no more emotion than that involved in flicking a cigar ash.

Today a man with such cold nerve might well be found in the movie business where the risks and the rewards are great. Instead of gambling on the turn of a card, they bet on the turn of the camera.

One of these men is David Lean, the British director who scored smashing successes with "Lawrence of Arabia" and "The Bridge on the River Kwai."

Today he is betting his reputation — and more than $10 million of MGM's money — that he can deliver an outstanding film based on Boris Pasternak's Nobel Prize novel, "Dr. Zhivago."

Although the book has been called "the least-read best-seller" and centers on the life of a Russian poet — not a commercial story, as the bankers would say — Lean and his cast are determined to produce a film that will be both an artistic and financial success.

The amount of talent involved is tremendous. Playwright Robert Bolt ("A Man of All Seasons") was paid $250,000 to do the screenplay in the hope that he could repeat his success on "Lawrence."

"David and I worked together for seven months on the Zhivago script," he said. "We almost came to grief several times, but everything finally worked out."

When it came time to assemble the cast, Lean again refused to play it safe and sign super-stars who supposedly provide insurance at the box office regardless of the film.

For the leading role, he picked Omar Sharif, the rising star who received an Academy Award nomination for his role in "Lawrence." Lean likes to work with young actors who do not feel they know more than the director, a common failing among some stars.

"The great challenge of the role is that Zhivago is essentially a passive man, yet a beautiful man," Sharif explained.

Although the story revolves around the Russian revolution and the rise of communism, it is focused on the characters caught in personal chaos rather than on war scenes. Here again Lean refused to fall back on an almost sure-fire box office technique.

FOR the role of Zhivago's wife, Lean selected Geraldine Chaplin, the 20-year-old daughter of the great comedian. Although she has received fantastic publicity, her acting experience to date has been a small part in a French film.

"If it was any other director besides David, I might think I was signed just because of my name, but David is a perfectionist and would never consider such a thing," the young beauty explained.

In selecting the actors and actresses for the character parts, Lean recruited some of the finest talent in the profession.

Rod Steiger, who has the inside track on an Oscar award for his exceptional performance in "The Pawnbroker," plays the ruthless opportunist who rides with the political tide.

Sir Alec Guinness, another veteran of "Lawrence," portrays Zhivago's half brother, while Sir Ralph Richardson is the wealthy aristocrat who helped rear Zhivago.

British actor Tom Courtenay plays the intense young revolutionist working 'to overthrow the czar, and Siobhan McKenna is the wife of Sir Ralph in the film.

To give the picture the proper set, a 1905 Moscow was built on the outskirts of Madrid, marble dust serving as snow for some scenes. The huge set even includes a trolley line rattling along for several city blocks and covers about 10 acres.

But, while the city scenes could be filmed under the warm Spanish sun, Lean and his group had to go elsewhere for the snow-covered fields and mountains of Zhivago's trek toward Siberia in the aftermath of the revolution.

FIRST the filmers went to Soria, one of the coldest areas in Spain, normally covered with snow each winter. But the snow didn't come at the right time, what did come melted and the rapidly changing weather made "matching" shots all but impossible for the cameramen.

Taking a firm grip on the corporate checkbook, Lean and his key people boarded a plane for a remote spot in northern Finland, where they spent two frigid weeks shooting the scenes that were originally scheduled for Soria. "There was certainly no shortage of snow in Finland," Sharif said.

The film is to be completed this fall, and the result will be of interest to many people besides Lean. Since this is the biggest MGM picture since "Ben Hur," the board of directors for Leo the Lion are counting the days until it is completed.

For the young stars, the picture could mean tremendous success that would have been years in the making if they had confined themselves to routine pictures.

Among professionals, the big question is, "Can Lean produce three smashing successes in a row?" One official of the company said, "David is the only man who could make this picture."

The "experts" of the movie industry are quick to offer opinions ranging from, "It's certain to be a great picture" to, "It's going to be the biggest bomb since Hiroshima." But the simple truth is that nobody can tell how successful a picture is going to be until it gets the final test — at the box office.

The chips are in place, and the wheel is turning as the cameras roll.

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