'One Night With Janis Joplin' is a lifelong relationship for star Mary Bridget Davies
It takes Mary Bridget Davies about an hour to physically become Janis Joplin. As for the mental and vocal preparation, "it happens throughout the day with me," says Davies, 34, sitting in her dressing room at Arena Stage. The star of the blockbuster hit "One Night With Janis Joplin," at Arena for the second summer in a row ("Back by popular demand!") and heading to Broadway in the fall, is wearing black leggings, a g…