When you’ve got big names like Jessica Lange, Michael Shannon, Gabriel Byrne and John Gallagher Jr. starring in “Long Day’s Journey Into Night” on Broadway, some equally big names will show up for the opening night.

At the April 27 opening of the Roundabout Theater Company’s revival, Jessica Chastain, Blythe Danner, Nathan Lane, Annette Bening and Rebecca Hall were among the recognizable faces in the audience. So were Ryan Murphy, who produced “Long Day’s Journey” in association the Roundabout, and Sarah Paulson.

Lange, of course, knows those two from her work on “American Horror Story.” But after the opening night performance of “Long Day’s Journey,” the actress said that her TV work over the last few years, intense as it was, doesn’t have much in common with the mammoth part of the drug addict Mary Tyrone in “Long Day’s Journey.”

“There’s nothing like this,” she said. “Nothing that I’ve ever done before.”

Over the course of the play’s four hours, Lange has to gradually disintegrate into a drug-addled stupor, seven times a week. It’s a marathon. “You don’t have a lot of life outside of this,” she explained. “You sleep late, you get up, you start immediately thinking about this show. Everything is gearing toward this. Everything is gearing toward the curtain.”

Byrne, appearing in his third Broadway production of a major Eugene O’Neill play, echoed that sentiment. “It requires focus and discipline and stamina and energy,” he said. “You have to really take care of yourself. You have to go to bed early and you have to eat properly. You have to exercise. And weirdly enough,” he added, “you have to keep your head clear of doubt and fear and all the demons that come out of the cupboard and try to diminish you. You have to keep a very clear connection to the play.”

Director Jonathan Kent put it more succinctly. “It’s a fiendishly hard play to do,” he said.