He himself is very unlike Darcy, he said, especially in the character’s belief in his own sense of authority. Mr. Kartheiser said he was eager for notes and pointers from his cast mates. “I’m not that good of an actor,” he continued, “so I kind of like say: ‘Yeah? How do I fix it? Was it that bad? O.K. let me try again.’ ”

Low-key and soft-spoken, he mixed actorly pronouncements (“there’s a real nurturing environment here”) and a courtly speaking manner (“the blessed role of Pete Campbell”) with sparing references to his personal life (his fiancée is Alexis Bledel, a guest star on “Mad Men”) and self-deprecating asides.

Unsure whether he could pull off Darcy, he said that every night backstage he’s thinking: “Is this actually happening again? Didn’t we just do this seven times last week?” But if the prospect of appearing before 1,100 people a night is nerve-racking, that’s part of the point.

This Minnesota-born actor grew up in theater here — his family is still in nearby Apple Valley — but between the ages of 15 and 25, he concentrated exclusively on television and film (landing leads in “Masterminds” and “Another Day in Paradise” and a recurring part in the series “Angel,” among other credits). By 2005, he realized he needed to “start pushing myself instead of what I had been doing, which was borderline complacency.”

So he signed up for the part of a British hustler in a dark comedy, “Slag Heap,” at the Cherry Lane Theater in New York. He received good notices (he “gives the play’s most accomplished performance,” wrote Charles Isherwood of The New York Times), and while the play itself didn’t fare well, the experience of performing for sparse, unappreciative audiences actually proved inspiring.

“You get off the stage and you go, ‘I’m still alive,’ ” he said.

“Pride and Prejudice” isn’t his first theater job since “Mad Men” began in 2007. Last summer, he starred in the premiere of a drama, “The Death of the Novel,” at San Jose Repertory Theater. But the new show does represent a return to his roots: He started acting here at the Guthrie, playing Tiny Tim in annual productions of “A Christmas Carol” and, as a teenager, in a cycle of Shakespeare’s history plays. He also logged significant time at Children’s Theater Company, a venerable Minneapolis institution, performing in, by his count, 18 main-stage shows and 3 national tours.