He became a legend as 'Bogie', the world-weary, wise-cracking outsider, but in reality Humphrey Bogart was plagued by doubts and demons. He was born upper-class yet made his name playing mavericks, drank with the rat pack and met four wives on set - including his great love, Lauren Bacall - yet always mistrusted stardom. Here David Thomson, one of film's most provocative writers, reveals the man behind cinema's greatest icon.