Sarah Ruhl was a poet before she was a playwright: She published a short chapbook at the young age of 20. But, in an interview with Poetry Foundation, she reflected, “I think playwriting was attractive [to me] because it’s multi-vocal; there are disguises, there are many characters.” Two in particular make sense in light of her days as a bard. Elizabeth Bishop and Robert Lowell, two iconic American poets, are the focus of Ruhl’s 2012 epistolary play, Dear Elizabeth. The show homes in on their decades-long postal correspondence. Through alcoholism and divorce, suicide of loved ones and stays in mental institutions, the pair exchanged more than 400 letters. Take a peek inside the envelope at the Dorset Theatre Festival staging of the show opening this week.