How does the absence of a narrator in Hedda Gabbler, a play (as opposed to a novel), affect the requirement for critical thinking in the viewer?
Your question seems to infer that the medium of the novel does all the work for the reader and does not force them to engage their brains in critical thinking in the same way that watching a play does. I would have to disagree if this is what you are saying. Both novels and plays, depending on how they are constructed, can force the reader or viewer to have to engage in them actively to pick up what the author/playwright is suggesting. You might like to think of the plays of Arthur Miller, particularly The Crucible, where before each character enters he gives lengthy introductions to them so as to put them in context and tell us about their various traits, as an example of a play where, in this sense at least, the need for active viewing is slightly diminished.
However, to talk about Hedda Gabbler, to pick one example that requires the audience to view the action actively, you might like to think about how Ibsen introduces the concept of duty and responsibility. We are never told as an audience that Juliana and Thea are used as foils for Hedda in this sense, as both show how these two qualities must emerge from loyalty springing from love as opposed to fear. Juliana is a sacrificial individual who is shown to be completely selfless. It is ironic that she annoys Hedda greatly, as Hedda is unable to understand why Juliana would give her life for those she cares for.
Thea is another sacrificial character, though in her case it is her willingness to sacrifice her reputation in terms of coming out of a marriage that is without love. Although the accepted thinking of the day at that time would be to judge her harshly for reneging on her marriage vows and rejecting duty and responsibility, Ibsen makes it clear through the action of the play and the dialogue that this is not the case. Consider her case: she was converted into nothing more than a drudge or a servant in her marriage. Her escape to Christiania to follow Lovborg shows that she possesses the kind of gumption and moral courage that Hedda does not have. Thea is ruled by love, whereas Hedda, it is shown, is ruled by fear.
Both of these contrasts are not directly told to us by a narrator. We are not spoonfed this knowledge by an omniscient narrator, and have to work this out for ourselves, becoming active viewers to establish, for example, the way that Ibsen champions Thea in the play.