P.T. Barnum always has been regarded as a great entrepreneur and show business pioneer who saw things no one else did, perhaps realizing the business was just one big circus. The new musical The Greatest Showman avoids the pitfalls of typical biopics to become more of a fantasia of song and dance, a joyous exercise in pure entertainment that is made for the holiday crowd. If it doesn’t get swallowed up by other family fare like another Jumanji, another Star Wars, another Pixar toon and even Fox’s own Ferdinand, audiences will discover than true rarity: an original movie musical. Although...