Despite a pronounced lisp, Karloff's stage-trained voice became an asset during the transition to sound. He scored his first success in The Criminal Code (1931), in which he repeated a previous stage role. But the real turning point in his career came later that year, when he was cast by James Whale in the role of the Monster in Frankenstein. Even the heavy makeup applied to Karloff's face could not hide the nuances of his performance. The film was a great success and assured Karloff a permanent niche in the horror film genre.
In contrast, Karloff was known as a mild-mannered, amiable gentleman who performed many acts of charity for needy children. Throughout his busy screen career (some 140 films in all) Karloff continued to return to the stage. He scored a great success in 1941 as Jonathan Brewster in the Broadway production of Arsenic and Old Lace and another in 1950 as Captain Hook in Peter Pan. He gave one of his best performances in one of his last screen roles, virtually playing himself, as an aging star of horror movies, in Peter Bogdanovich's Targets (1968).
Biography from Katz's Film Encyclopedia