Brent Lang Executive Editor Juliet Stevenson seems almost ashamed. There was a moment a few weeks ago when the boundaries between Ruth Wolff, the gifted but abrasive physician whose downfall drives the action of “The Doctor,” and Stevenson became too porous. “I crossed this line and didn’t know who I was,” Stevenson says on a recent afternoon at the Park Avenue Armory, where she has been performing in the play since June.