Kelly (Rachel Zampelli) lets Peter (Thomas Keegan) read a letter from his dead twin brother, her husband (also played by Thomas Keegan).

Some actors find two-character plays especially brutal, because they rarely get emotional or physical breaks during the show. Thomas Keegan is way more hard-core than those people.

“Dying City,” running at Signature Theatre, has three characters and a cast of two. As twin brothers Craig and Peter, Keegan is onstage for nearly the whole play. He even switches roles without leaving the set.

Peter, an actor, is still reeling from the death of his twin. Craig was a soldier in the Iraq War, stationed at Abu Ghraib, who had trouble communicating with his wife, Kelly (Rachel Zampelli). The play is a 70-minute power struggle among the three characters, Craig in flashback, Peter in the present.

Keegan had some invaluable help understanding the relationship between Peter and Craig: Director Matt Gardiner has a twin brother himself (actor James Gardiner).

“I have a feeling of ownership over my twin brother,” Gardiner says. “His triumphs are more thrilling to me than any of mine. And when he falls, it is incredibly painful.”

Craig and Peter are disgusted by and obsessed with each other’s choices. They subtly mirror the dynamic between Americans at home and troops in Iraq, who are seen at once as liberators and conquerors, as fighting terror and propagating it.

In playing twins, there’s a temptation to make it obvious to the audience which character is on stage. Gardiner and Keegan decided that overemphasizing the differences between Craig and Peter could make the double casting come off as a gimmick.

Keegan’s dual performance is informed by the quirks of playwright Christopher Shinn’s script. For example, Shinn writes that Peter used to impersonate Craig when they were children. It’s a useful detail, given that Peter grew up to be an actor, and a handy explanation of the characters’ similar mannerisms.

“The twins are two distinctly different people who have different energies and different motives and tactics,” Keegan says. “But there are moments where it will become somewhat unclear to the audience exactly which brother Kelly is dealing with,” and that’s by design.

Says Keegan: “If the audience isn’t quite sure who I am, that’s OK with me.”

The Story: Kelly (Signature regular Rachel Zampelli), a therapist, is mourning her husband Craig’s mysterious death in the Iraq War when Craig’s twin brother, the somewhat unstable Peter, shows up and wants to talk about his feelings.

Signature Theatre, 4200 Campbell Ave., Arlington; through Nov. 25; 703-820-9771.