"How do you differentiate good acting from bad acting?

"
appeared as a question on Quora. Below we are printing one of
the top answers.
If anyone tells you there are objective standards, they're full of s-t. This is a matter of personal taste. There are trends. There are
many people who loved Philip Seymour Hoffman's acting. But if
you don't, you're not wrong. At worst, you're eccentric.
(An interesting question and one you didn't ask so I won't
answer it, here is why are there trends? Even if Hoffman
isn'tobjectively a great actor, why do so many people love him?
For that matter, why do so many people love the Beatles,
Shakespeare, and Leonardo Da Vinci? Maybe someone will ask a
question about why there are general trends in taste ...)
I'm a director who has been working with actors for almost 30
years, and I'm the son of a film historian. I'll give
you my definition of good acting. But I really want to stress (for
the last time, then I'll quit) is that if I say Pacino is great and you
disagree, my experience does not make me right and you wrong. It
just means we have different tastes.
For me, an actor is good if ...

1. He makes me believe he's actually

going through whatever his character is
going through.
I'm talking somewhat about physical stuff ("He really is getting
shot!" "He really is jumping off a moving train!") but mostly about
psychological stuff. ("He really is scared!" "He really is in love!") If

an actor seems to be "faking it," he's not doing his job (as I define
it).

2. He surprises me.
This is the most nebulous requirement, but it's important. Except
for really small parts that aren't supposed to call attention to
themselves (e.g. a bank teller who just cashes the hero's checks),
it's not enough for actors to just seem real. Seeming real is a
requirement, but a second requirement is that I can't predict their
every reaction before they have it.
Think of how a woman might react if her boyfriend breaks up with
her. There are many, manytruthful ways ways which would
seem like a human being reacting and not like a space alien
behaving in some bizarre, unbelievable way.
She might break down and cry; she might laugh hysterically; she
might throw water in his face; she might go completely numb,
having no expression at all ...
An actor's job is to know the breadth of human possibility and the
depths of their own possibilities. They must pull from this well
and surprise us. Otherwise, they become boring and predictable.
There are many ways and actor can surprise. Gary Oldman and
Johnny Depp surprise us by being truthful while playing multiple,
very different roles. Jack Nicholson surprises by being ...
surprising. Even though he's not a chameleon like Oldman or
Depp, you never know what he's going to do next. But whatever
her does, it's grounded in psychological reality. It never seems
fake.
Christopher Walken, Glenn Close, Al Pacino, and many others
have a surprising danger in them. They're a little scary to be

around, because you feel they might jump you or blow up at you at
any time. They are ticking time bombs.
And, of course, many comedic actors (e.g. Julia Louis-Dreyfus)
surprise us in all sorts of quirky, zany ways. Or watch Katherine
Hepburn and Cary Grant in "Bringing Up Baby." Absolutely
surprising and absolutely truthful!
Another great example of surprising acting that never seems fake
is Diane Keaton's work in "Annie Hall."

3. He is vulnerable.
Great actors share the parts of themselves that most people keep
hidden. They are always naked. (Some are literally naked, but I'm
talking about emotional nakedness.) Bad actors are guarded. They
don't want to share the parts of themselves that are ugly, mean,
petty, jealous, etc.
There are so many examples of actors being naked on stage and
screen. My favorite is Rosalind Russell in the movie "Picnic." Rent
it some time if you haven't seen it. She plays a middle-aged
schoolteacher who is in danger of growing old and dying alone.
There's a heartbreaking scene in which she begs a man to marry
her. She goes down on her knees in front of him. She gives up
every scrap of dignity inside her and lets the scared, hurting parts
of herself burst out.
These are the same scared, hurt parts that are inside all of us
the parts we work hard to hide. Hiding them (by holding them in)
takes a toll on us, and one of the greatest gifts actors can give is to
sacrifice their dignity for us for us. They expose themselves so we
don't have to. They are like Christ dying for our sins.

This ties in with everything I wrote above: when actors are

exposed and raw, it's always surprising. And if it doesn't seem
real, there's no point in it. In fact, this sort of emotional nakedness
is very hard to fake. If you ever get a sense that an actor is showing
you a secret part of himself, he probably is.
Examples (in my opinion) are Julianne Moore and Bryan
Cranston. Also, rent "The Browning Version" sometime. The early
one (not the remake). Watch Michael Redgrave. He turns himself
inside out and wrings out all his pain.

4. He knows how to listen.

It's fascinating to watch actors when they're not speaking. Some
are too caught up in ego or technicalities (e.g. trying to remember
their next line) to totally focus on whoever it is they're acting with.
Others seem to register everything they hear. You can see
whatever is being said to them physically affecting them, as if the
words are slapping them across the face. Watch Claire Danes.
She's an amazing listener.

5. He has a well-honed "instrument."

By which I mean he knows how to use his voice and body to serve
whatever role he's playing. This doesn't necessarily mean he's slim
and has a six-pack. James Gandolfini used his body well. It means
he knows how to move and talk in expressive ways. His voice and
body aren't fighting him or holding tension that's inappropriate to
his role.
One negative example: Kristen Stewart. It's almost painful to
watch her. She looks like she'd rather be anywhere else besides in
front of a camera. She is (or seems) very self-conscious.

To me, Hoffman was great because he embodied all of these traits.

He was vocally and physically gifted. He wasn't in great shape, but
he used the shape he had in expressive ways. If you watch him
closely when he's not speaking, you'll see he always listened to his
co-stars closely. What they say affected him deeply, and his
reactions grew organically out of whatever they had previously
said or done to him.
He was profoundly vulnerable. Always. This was his most
distinctive trait. You always knew what you were getting from him
was raw and honest. It was this rawness as well as intelligence
and a sly sense of humor that made his work surprising and
fresh. And I never once saw anything from him that seemed fake.
I don't hate Tom Cruise the way some people do. To me, he's
believable most of the time. He's just not very interesting. He
rarely surprises me, and he doesn't seem to dig deep into anything
raw or vulnerable inside him. He seems guarded. The must
vulnerable I've seen him is in "Eyes Wide Shut," in which he did
some good work. But it wasn't brilliant. And it's not his norm.
Keep in mind that many people (who aren't themselves actors,
directors, or obsessive film buffs) aren't very clear on what an
actors contributes to a film. Which is fine. It's not necessary for
most audience members to understand who does what during
production.
Lots of people think an actor is great if they like his character. But
that's often a function of good writing more that good acting. Or
they think he's good if he pulls off some impressive effect, such as
gaining or losing a lot of weight or pretending to be handicapped.
Those are impressive stunts, but they aren't the core of what
actors do. If you forced me to rank Dustin Hoffman in "Rain Man"
vs. Dustin Hoffman in "Kramer vs. Kramer," I'd say he did more

exciting work in the latter. In "Rain Man" he was able to hide

behind some stunts. In "Kramer vs. Kramer," he just had to be
truthful.
Some people think acting is good if they like the movie. Keanu
Reeves, in my mind, is a horrible actor mostly because he's
wooden and fake. It often seems as if he's reading from cue cards
rather than saying words that are his. But some people like him
because they think the Matrix films are cool. They confuse the
movies with the actor. If some other actor had been in those films,
those same people would have liked him. It's not really the actor
(or not entirely the actor) they're liking. But since he plays the
protagonist, they focus on him.
Finally, many people confuse an actor's life with his work. Tom
Cruise is a good example. He's a high-profile Scientologist, and
many people dislike that religion. They dislike his acting at least in
part because they find him unsavory as a person. To some extent,
this may be a sign of bad acting on his part. At least, he's not a
good-enough actor to make people forget about his private life
while they're watching him in movies. To some extent, it wouldn't
matter how skilled he was.
Currently, many people are having strong reactions to work by
Woody Allen and Mia Farrow that have nothing to do with what
they're doing on screen. I'm not even remotely saying such people
are wrong, stupid, or crazy. I'm just saying that people's reactions
to actors are often complicated and not 100% influenced by their
performances.
UPDATE: A couple of people have asked me to elaborate on my
claims about Keanu Reeves. They feel that although he's often
wooden, this is appropriate for his character in "The Matrix." I
will admit up front that I only saw the film once, when it first

came out, so it's possible I'm misremembering. Certainly, a good

director can sometimes put bad actors to good use.
Let me confine my remarks about Keanu to his acting in general,
not just in "The Matrix," though I am still skeptical about his work
in that movie.
There is a difference between playing an undemonstrative person
and being a wooden actor. In fact, playing someone who is
reserved is very difficult (because you have to act without showing
very much), and the actors who pull it off are brilliant.
I would point you to Anthony Hopkins in "Remains of the Day,"
Tommy Lee Jones in many of his roles, and even Clint Eastwood
in "Dirty Harry." These actors manage to convey the sense that
though they have stony exteriors, there's much going on
underneath. To me, Keanu Reaves conveys an actor who is
showing up and saying his lines. I don't believe much else is going
on underneath except maybe nervousness. If you feel otherwise,
that's fine. Remember, it's subjective.
Having auditioned many actors, I'm used to hearing ones that can
take any writer's lines and make it sound like their own words.
And I'm also used to less experienced (or less gifted) ones who
sound uncomfortable with words that aren't their own. They
sounds as if they're arereciting or reading something. They
sounds scripted.
Listen to Keanu in the clip, below, especially at around 10-seconds
in, when he says, "I have offended you with my ignorance, Count."
Many of his line-readings sound like that to me. He has not fully
lifted them off the page and into his own mind and body.