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Charles Frohman, Constance Adams, and David Belasco

Cecil B. DeMille began his career as an actor on the stage in the theatrical company of Charles Frohman in 1900. He debuted as an actor on February 21, 1900, in the play Hearts Are Trumps at New York's Garden Theater. In 1901, DeMille starred in productions of A Repentance, To Have and to Hold, and Are You a Mason? At the age of 21, Cecil B. DeMille married Constance Adams on August 16, 1902, at Adams's father's home in East Orange, New Jersey. The wedding party was small. Beatrice DeMille's family was not in attendance, and Simon Louvish suggests that this was to conceal DeMille's partial Jewish heritage. Adams was 29 years old at the time of their marriage, eight years older than DeMille. They had met in a theater in Washington D.C. while they were both acting in Hearts Are Trumps.

They were sexually incompatible; according to DeMille, Adams was too "pure" to "feel such violent and evil passions". DeMille had more violent sexual preferences and fetishes than his wife. Adams allowed DeMille to have several long-term mistresses during their marriage as an outlet while maintaining an outward appearance of a faithful marriage. One of DeMille's affairs was with his screenwriter Jeanie MacPherson. Despite his reputation for extramarital affairs, DeMille did not like to have affairs with his stars, as he believed it would cause him to lose control as a director. He related a story that he maintained his self-control when Gloria Swanson sat on his lap, refusing to touch her.

In 1902, he played a small part in Hamlet. Publicists wrote that he became an actor in order to learn how direct and produce, but DeMille admitted that he became an actor in order to pay the bills. From 1904 to 1905, DeMille attempted to make a living as a stock theatre actor with his wife, Constance. DeMille made a 1905 reprise in Hamlet as Osric. In the summer of 1905, DeMille joined the stock cast at the Elitch Theatre in Denver, Colorado. He appeared in eleven of the fifteen plays presented that season, although all were minor roles. Maude Fealy would appear as the featured actress in several productions that summer and would develop a lasting friendship with DeMille. (He would later cast her in The Ten Commandments.)

His brother, William, was establishing himself as a playwright and sometimes invited DeMille to collaborate. DeMille and William collaborated on The Genius, The Royal Mounted, and After Five. However, none of these were very successful; William deMille was most successful when he worked alone. DeMille and his brother at times worked with the legendary impresario David Belasco, who had been a friend and collaborator of their father. DeMille would later adapt Belasco's The Girl of the Golden West, Rose of the Rancho, and The Warrens of Virginia into films. DeMille was credited with creating the premise of Belasco's The Return of Peter Grimm. The Return of Peter Grimm sparked controversy, because Belasco had taken DeMille's unnamed screenplay, changed the characters, and named it The Return of Peter Grimm, producing and presenting it as his own work. DeMille was credited in small print as "based on an idea by Cecil DeMille". The play was successful, and DeMille was distraught that his childhood idol had plagiarized his work.