How do you convince a marquee entity like Tom Hanks to remake a dark 1955 British comedy about a heist mastermind out to murder an old lady who's on to his scheme? Simple: Send him the script. ''It was that easy,'' says Ethan Coen. Adds brother Joel, ''He read it and gave us an answer. There wasn't a lot of Sturm und Drang.'' There wasn't a lot of parsing the Alec Guinness original, either: The Coens say Hanks didn't want to see it as he put together his character, Prof. Goldthwait Higginson Dorr III (the names and personalities depart from the British film), a grandiloquent know-it-all with a plan to tunnel into the counting room of a Mississippi riverboat casino. The Coens call Dorr's draped-cape look ''a cross between Colonel Sanders and Edwin Booth,'' and say Hanks hammered out his drawly speech patterns by watching old ''Firing Line'' tapes of William F. Buckley. His teeth, too, were made ''a little snaggly,'' as Ethan describes it. ''Southern dental care.''