It is superb acting. The writing is incredible. It must have been like heaven having to recite the dialogue.
Peter Finch won the Best Actor Oscar. I'm glad either he or Holden won it. Finch has the more iconic lines. I slightly preferred Holden.
I'm even happier to see Dunaway won for Best Actress.
I love the first Rocky film, but either Network or All the President's Men should have taken home Best Picture. Both films would make my top 10 films of the 70s.

With Chayefsky's script, they probably had their pick to play every speaking part. Practically every character gets at least one show-stopping monologue. What actor wouldn't have been clamoring for a chance to chew on that dialogue?
Ned Beatty's time on-screen is the most darkly funny five minutes or so in movie history.

I have meddled with the primal forces of nature and I will atone.

I agree with you. The acting was SO believable. It was superb! It is true most of the time you don't find this many terrific performances in just one film. The director did a great job.