Albie Hay’s review published on Letterboxd:
One of the slipperiest films ever made - as soon as you've settled on one "explanation", another one presents itself, and then another, and before long you're scampering all over the place trying to collect them up and arrange them into a "theory" about what you've just seen. It's often referred to as a film about cinema, but something that occurred to me during this, my second viewing, was that it perhaps explores not only cinema but the arts in general, using the character of Elisabet as a repository for its ideas about the arts. Perhaps her refusal to speak represents the impossibility of forming a personal connection to a work of art, which perhaps translates into a refutation of Roland Barthes' Death of the Author: we can project all the thoughts and feelings we want onto a film or a novel or a piece of music, but ultimately it will remain an unchanging, inanimate object, inactive and unresponsive to our attentions. Or maybe not: when Elisabet treads on the broken glass laid by Alma, the film appears to rupture. So perhaps Bergman is saying that, in this post-structuralist era, we do in fact have the power to alter a text's meaning. In this context, the increasing determination with which Alma impresses her own personality onto Elisabet (see the repeated monologue) maybe suggests a need to wrest control of the act of interpretation away from the author - something that, once achieved, causes all objective meaning to disintegrate, hence "Nothing" being the only thing Alma is able to make Elisabet say.
Or maybe it's a film about lesbian vampires.