Joseph Cotten led two lives. On Broadway, the Virginia-born actor, who died of pneumonia on Feb. 6 at the age of 88, was a genuine star, beginning with his 1939 portrayal of The Philadelphia Story's C.K. Dexter Haven (Cary Grant played the role in the movie). On film, though, Cotten created a gallery of nice guys whose decency was inevitably swamped by uselessness. Early in his career, he hitched his star to that of Orson Welles (they met when both were tossed out of an audition), who cannily used Cotten as a lesser mortal to his own charismatic genius. Cotten didn't seem to mind, perhaps reserving ego for Broadway. ''I didn't care about the movies,'' he once said. ''I was tall. I had curly hair. I could talk. It was easy to do.'' Three that defy his own dismissal: *Citizen Kane (1941, Turner) Cotten is the defeated conscience of the film as Jed Leland, the hapless friend and partner of Kane (Welles). *Shadow of a Doubt (1943, MCA/Universal) His rare killer lead as Uncle Charlie, the sweet-faced Merry Widow Murderer in Hitchcock's favorite of his own films. *The Third Man (1949, Republic) As Holly Martins, he was a symbolically naive American lost in the dark moral thickets of postwar Europe.