My feeling is both acted normally instead of neurotically and/or idiosyncratically (which tends to be the norm), and since the director veered away from the Oedipal/incest controversy which wrecks many stage and screen adaptations, the normalcy of their performances enabled the natural flow of the play to emanate Shakespeare's intended course.
Claire Bloom acted like a dignified, noble, honest woman who was trying to hold her head up knowing that she committed a grave error by re-marrying too rashly, and it was clear she never once thought Hamlet (her husband) was murdered by Claudius, and her behaviour strictly adhered to Shakespeare's written word.
Polonius is portrayed according to the written word, as your average conniving person looking out for his best interests, but not really interested in harming anyone. Polonius is also trapped in a deadly situation. He's worried about the fate of his daughter, since he probably expected Ophelia to marry Hamlet and become a princess, and eventually queen, which is negated by what Claudius has done, and he's likely aware that Claudius killed his king and that Claudius may suspect him of knowing and could at any moment order his death. Polonius is also fully aware that Hamlet is carefully planning to bring down Claudius. He's everywhere in the play, trying to balance the unbalanced in order to maintain the life he knew before King Hamlet was murdered, but he's outwitted at every turn by a clever philosopher-prince who never liked Polonius before and liked him even less now that he's kow-towing to Claudius. Polonius is an extremely wise man, but he doesn't take his own advice and gets trapped in his own vice.
Because Eric Porter is given the opportunity to portray Polonius per Shakespeare's own words, we sympathize with Polonius, we understand his dilemma, we even root for him, and because of this we believe Hamlet when he laments killing Polonius.
Fantastic production, but still not as good as Branagh's...