In an arguable and interpretive, not necessarily the only right answer, assertion, what could be Margrethe's symbolic role in Frayn's Copenhagen?
Margrethe: A talk? To the enemy? In the middle of a war?
Bohr: Margrethe, my love, we were scarcely the enemy.
Margrethe: It was 1941!
Bohr: Heisenberg was one of our oldest friends.
Margrethe: Heisenberg was German. We were Danes. We were under German occupation.
Bohr: It put us in a difficult position, certainly.
Margrethe: I've never seen you as angry with anyone as you were with Heisenberg that night.
Bohr: Not to disagree, but I believe I remained remarkably calm.
Margrethe: I know when you're angry.
Act I sets Margrethe up in a dual role, that of character and that of symbolic character. While she is Bohr's wife and confidant, therefore used to engaging with him, she is also represented as the voice of reason; as the voice of practicality; as the voice of insight. When all of these are combined, hers becomes the narratorial voice, the symbolic voice of omniscient vision.
In Act I, Margrethe is shown to be the one dependent upon and espousing reason.
Margrethe: It was 1941! [...] We were under German occupation.
As the voice of reason, she is the one who sees reality. She asserts facts in the face of impressions and fond hopes. As the voice of practicality, she directs attention toward danger and threats. She directs attention toward protective cautions. She attempts to warn and instruct:
Margrethe: I think you must also assume that you and I aren't the only people who hear what's said in this house. If you want to speak privately you'd better go out in the open air.
As the one with insight, she perceives and understands others' feelings and actions and interprets them rightly, while other characters may underestimate them.
Margrethe: They so much wanted to see each other, in spite of everything! But now the moment has come they're so busy avoiding each other's eye
In essence, one might say that Margrethe is the narrator of the work, the one able to see and comment on the inner workings of Bohr's and Heisenberg's minds and feelings.
This is an interesting technique of character development because Margrethe is not only one of the central characters herself, she is an active member of the flashback that comprises the heart of the play, a flashback Margarethe helps introduce and navigate through her objective and insightful commentary, thus continuing her narratorial type commentary in the flashback:
Margrethe: Another silence. He's done his duty.