I would say that Groucho Marx was a great entertainer (which is no small accomplishment) but I wouldn’t call him a great actor.
Having just watched ‘The Father’, here are some of my reasons for calling Sir Anthony Hopkins a great actor.
For those of you who don’t know the film*, it follows Anthony Evans, a Welsh man who is dealing with dementia. One of the great features of the film is that it presents the world through Anthony’s point of view, leaving it to the viewer to try to objectively put the pieces together into a fully coherent story. The wiki article is here - The Father (2020 film) - Wikipedia
One of Sir Anthony’s greatest challenges in the film is that he must respond to his surroundings and his fellow actors in different ‘wrong’ ways, often in the same shot. It isn’t just that his character doesn’t understand what is going on, it’s that we see him struggle to understand and come up with a subjective conclusion. He has to go from charming to irascible to vulnerable, turning the emotions we perceive on a dime.
There is a specific exercise that actors work on constantly - I know it as ‘endowment’. This is when you have to treat an object, a set, or a fellow actor as something that they’re not - for instance, the glass you’ve just been handed is supposed to be a fabulous piece of expensive crystal, when in fact, it’s something the props department picked up at the dollar store. Or, you’re supposed to be in Versailles when it’s just a warehouse that’s been tarted up. Or, your scene partner is supposed to be the Queen of all Russias, when in fact, she’s Cassie from Perth and swears like a sailor in real life.
Anyway, Sir Anthony has the challenge of endowing objects with being the wrong object. As but one of many potential examples, we see him with his glasses, then he picks up a fork as if it were his glasses. Then when his daughter finds the fork in his collection of precious objects, she’s baffled as to why that got put in there - but we the audience know, and we have an emotional attachment to that fork because we saw what he was mistaking it for.
It’s a film I intend to watch many more times before I have to give it back to the library, but here’s the thing - as an actor, I’m aware of these things and I’m looking for them in everything I watch. Watching this film for the first time, I completely gave myself over to the story and the characters - it was on the second viewing that I started taking mental notes of Sir Anthony’s specific techniques.
I don’t want to say I could never accomplish what he did, but let’s be honest - I don’t have the acting chops nor the experience in film to come anywhere close to his performance! This film was originally a play, which has me thinking ‘How in Hell could they have done all the brilliant set work, where the kitchen changes from one shot to the next, on stage in real time?!? How could the actor playing Anthony make his way through all those emotional and mental shifts in one night live?’.
And then, I think of Sir Anthony in ‘The Bounty’, or in ‘The Silence of the Lambs’, or in ‘The Remains of the Day’, and I think ‘The same actor played all those parts, and he was utterly believable.’.
He won ‘Best Actor’ at the Oscars that year, and ‘Best Actor’ at the BAFTAs, so to the extent that the accolades of his peers can be considered ‘objective’, there is something there in his acting that many people found admirable - it’s not just me!
*A warning to anyone wanting to see the film - if you have or have had dementia sufferers in your circle of family and friends, be forewarned! Even though the film does not plumb the utter depths of how hard it can be to look after a dementia patient, it is still realistic enough that it’s very hard to watch!
Gary Oldman was going to be my standard. For objectivity, you could show a group of people a series of movies, and then provide a questionnaire to determine how many of them realized it was the same actor in different roles, and how long it took them to realize that. The more people who didn’t realize it at first, and the longer it took them to finally realize it, the better the actor.
Well, as an art teacher, I’ve always taught my students to critique any form of the arts:
“How well did the art, the music, the writing, the acting, the work do what the artist intended it to do?”
I watched The Father with my partner a few months ago on Netflix. We were both in tears by the end of a superb film, and Hopkins’ performance was magnificent. In fact all the actors were excellent. I would say that provoking an emotional response in a viewer is a sign of a good performance. Presumably microexpressions in actors’ faces are something that we pick up? Perhaps lesser actors can’t achieve these?
I did something like this in a Eurovision-style music contest on a forum a few years back. There was one song I absolutely hated, hated hated! But since I was pretty sure the artist wanted me to hate it, I ended up giving it a pretty good score, for being so effective.
I hardly ever see movies, and I’m terrible at faces (one of the reasons why I rarely see movies), and I am not at all up on movie actors in general (I don’t know who Gary Oldham is, for instance), but my story is that I went to see the movie Doubt when it came out. I thought it was very good, and I was quite impressed with the woman who played the older nun. Sister Aloysius. “Who played that role?” I asked my wife on the way out of the theater. “Anyone I’ve heard of?” She looked at me curiously. “Meryl Streep,” she said.
Oh! I knew who Meryl Streep was and I’d seen her in a couple of other things, and I had NO IDEA that was her. She moved seamlessly–and from my perspective, anonymously–into the part, and I find that to be the mark of a fine actor.
Relatedly–Count me also as another who doesn’t think of Groucho Marx as a great actor, or even a particularly good one. Far as I’m concerned, he always played…Groucho Marx. Which he did better than anyone else, I suppose, but that doesn’t seem to indicate greatness. Just my two cents.
We all act and react in our own unique ways in real life - someone says something stupid and we roll our eyes. We call in sick and try to make sure the boss hears how sick we are. Someone says something funny and we laugh.
The trick in acting is to make it fresh every performance, every take, every time. And that’s what separates the sheep from the goats (or the GOATs - 
).
To get ridiculously actor technical - Sir Anthony Hopikns always ‘physicalizes the moment of revelation’, which is a fancy way of saying you can always see the moment in which his character gets what someone just said, or when his character figures something out. I remember in ‘Remains of the Day’, he was in extreme close up, and it was just a change of focus in his eyes. Nothing else moved, but you knew ‘he got it’.
My Google-fu is weak today - there’s an iconic scene from “Heat” featuring Al Pacino and Robert deNiro in a diner. Someone, somewhere, had found the same scene shot with two different actors, and watching the same scene twice with a different cast showed how incredibly well Pacino and deNiro lifted that scene off the page. I found the Pacino/deNiro version in less than a second, but damned if I can find the other version…
Technically its subjective but good objective arguments can be made.
To answer the thread title: In theory, yes.
Use motion capture to record an actor’s performance of various scenes and as various characters. “You’re an old man from Ireland whose dog died.” “You’re a young black girl who wants to go shopping.” “You’re a used car salesman from Kentucky suffering from alcoholism, trying to convince your wife that you’re not cheating on her.” And so on.
You now replay the performance with the appropriate body and voice, transformed via AI.
Do this for all actors that you want tested and form a panel that votes on each performance.
Repeat with more panels, with different makeups - professional actors, regular folk, old Irish men, young black girls, used car salesmen from Kentucky, etc. If you get similar rankings from multiple, diverse panels then you have a complicated but reasonably objective way of ranking actors by acting ability.
How do you grade them - by how convincing they are, how affecting they are, or how entertaining they are? What about the actor who can’t nail the Kentucky accent, but is absolutely riveting as a guy cheating on his wife? What about the actor who can only do the old Irishman, but does it perfectly? What about the actor who’s good at all of them but not the best at any? Which one of them is the best actor?
I think the scientific approach is to specify which thing you’re measuring, what instructions you gave to the panel, and it’s on the reader of your research to decide if your measurement applies to the question in his own mind.
We might imagine in oldentimes that someone wanted to know about the measurement of “oomph” of a projectile. Some scientists created reports on force, others on deflection, and yet others on blast radius and hole size. A lazy science reporter might have lumped all of those under the heading of “oomph” when they shouldn’t have. That’s the fault of a journalist and a reader, not the scientist or the methodology. But any of those measurements would still be a proxy for what the reader wanted, even if it might not be the best attuned to what the reader wanted.
That there’s different measurements of “oomph” doesn’t make measurements of force and deflection unscientific.
So? They would answer subjectively and very well might react/respond differently a few years later.
I’m struck by the parallel to sports that involve judging - is there an objective way to decide if someone’s a good figure skater? Or is it all subjective?
So far, we attain a type of objectivity by seeding the judge’s panel with mutually opposing self-interests. “And that’s a 4.1 from the Russian judge…”.
There’s certainly subjectivity there, particularly around artistic expression, but a double axel is a double axel, and my understanding is that most of the judging is around whether the skater correctly performs the jumps, spins, and (in pairs) throws that are part of their routine.
Trainers have “always” analyzed the efficiency of athletes’ movements. AFAIK, for decades, they’ve been using high-tech applications to do this, which suggests objectivity.