Holbrook, raised in Cleveland, was 29 when he first performed as Twain (who was portrayed as 70) and eventually developed the role into a two-act, one-man show called “Mark Twain Tonight!,” taking it to schools, nightclubs and theaters. He took it to Broadway three times — 1966, 1977 and 2005 — and won a Tony Award as best dramatic actor for the 1966 version. In 1959, after years of honing his material in small towns, Holbrook debuted his Twain at an off-Broadway theater in New York to high critical praise. “Mr. Holbrook’s material is uproarious, his ability to hold an...