Can you trust an actor?

Joaquin Phoenix went on David Letterman’s late-night show in 2009, and things got weird real quick.

Phoenix wore a beard and sunglasses, and barely answered questions. To cut the awkwardness, Letterman eventually asked him, jokingly, about his time with the Unabomber.

Turned out, the Academy Award-nominee was actually acting the whole time, and Letterman didn’t know.

If there’s an actor in your life, should this kind of thing give you pause? Are actors bringing their skills into real life, to fool us?

Writer-comedian and actress Addie Weyrich did just that, and got away with it.

You may have seen her on the HBO show “Crashing,” where she was a friendly Frisbee player in Central Park. Or on “The Marvelous Mrs. Maisel,” where she was an eager resort worker. She was in a Super Bowl commercial this year, helping some Legos with groceries at Walmart. You can catch her in a skin-care commercial making the rounds, too.

“So if you see a brunette lady who says, ‘When I had acne, I made up a lot of excuses not to go out,’ that’s me,” she said.

Weyrich lives in Los Angeles, but she was home for a visit in Philadelphia last year. And she was hiding something.

“I have this memory of being a little girl and my mom being like, ‘Listen, you can pierce your ears as much as you want but just please no piercings on your face, and no tattoos,’” she said.

No face piercings, but Weyrich broke that second prohibition. She was in Sweden studying theater improv, and to commemorate the experience, and give a nod to Philly sports, she decided on a small tattoo.

“I got ‘trust the process,’ but it says, `lita på processen,’ which means ‘trust the process’ in Swedish,” she said. “And I got it tattooed under my right boob. So very… easy to be hidden.”

She was home for the holidays and successfully hid “lita på processen” for two months.

But then one day, she was getting dressed and chatting with her mother in her bedroom.

“We’re not like a naked family in a weird way,” she said. “I’ve changed in front of my parents, especially in front of my mom.”

And Weyrich remembers the tattoo mid-change. She tries nonchalantly turning away from her mom to hide the ink.

“And she stops what she’s saying, and she’s like, ‘Hey, what’s that? What’d you do?’ And I was like, ‘Oh, what? I’m just putting my shirt on,” Weyrich said. “And she was like, ‘Why’d you turn around?’ Like she caught me immediately. She’s a journalist, she has a crazy hawk-eye.”

Mom is Ronnie Polaneczky, a journalist at the Philadelphia Inquirer. She wasn’t thinking tattoo, but she knew something was up.

“I thought, did she have an injury? You know, did she have a fall? Did someone hurt her?” Polaneczky said. “I mean, I didn’t know because it was so not like her.”

For Weyrich, it was showtime.

“This is so bad,” she said. “I was like, “Mom, I just like, I feel really bad about my body right now, and I just don’t want anyone to look at it! I don’t want you to look at it! I just don’t like my body right now, so just don’t look at it!”

For your consideration: Addie Weyrich as insecure daughter at the end of her rope.

“And I did it so well that I was crying and really believing it,” Weyrich said.

And her mother?

“She was like, ‘I’m so sorry. Like we can, oh my God, I’m so sorry,’” Weyrich said.

It worked. Her mom believed her performance.

“I won. I got it,” Weyrich said. “She did not look again.”

It worked maybe a little too well.

“I think she even managed to get herself to cry a little, and she made me feel really guilty for even asking or noticing,” Polaneczky said. “And I thought, you know, she’s a young woman and, they get horrible messages about their bodies and I guess that’s what’s going on.”

Good actors make us believe them all the time. It’s their job.

Marlon Brando really is heartbroken, yelling out for his wife in “A Streetcar Named Desire.”

Jack Nicholson is actually furious, screaming down the prosecution in “A Few Good Men.”

But all that happens on screen. How often do acting skills extend into the real world?

Gregory Donaldson used his acting training on a New York City subway to stay safe. He was writing about gangs in the late 1980s, when the homicide rate was astronomical.

“I would have a bit of a sour look on my face, and it’s got to do with posture as well,” he said.

If there were menacing teenagers on the train, he wanted them to think: That guy is probably a cop.

“I would make eye contact with the kids. And the look I had on my face was one of that I was somewhat bored, amused, but never frightened,” he said.

These little changes in posture or facial expression — actors call them adjustments. Little tweaks that make you into a different person. These days, Donaldson teaches a theater and criminal justice course at John Jay College of Criminal Justice in New York. Does little scenes with students, some of them aspiring cops.

Decades ago, he studied under the famous Sandy Meisner, who developed a type of method acting known as the Meisner Technique. Jeff Goldblum, Diane Keaton and other stars studied under him.

A hallmark of the technique is repetition: You just keep saying and listening to the same line over and over.

“It begins to become apparent to you as you’re doing it, that the words are meaningless,” Donaldson said. “That it’s the feeling beneath the words.”

He was in a scene in one class where his wife comes home in the middle of the night. His line was: “Where were you?”

“And then on the third or fourth or fifth ‘Where were you?’  the anger was [growing]. I was out of my chair, and I was ready to throw the table across the room,” he said.

The repetition is meant to teach an actor to listen, really listen, and react in real way. He also tries to tap into what he calls his emotional memory.

“[It’s] that reservoir of your real feelings, your real reactions, your real emotions,” Donaldson said.

If actors are good at tapping into emotional memory, can they just do it in everyday situations, like, for instance, to lie?

Even though he got away with his cop act on the subway, Donaldson said acting is an art, not a kind of deception. Great actors use some kind of truth to make an imaginary situation convincing, he said.

An expert on lying agreed.

“I would argue that good liars are pretty good actors most of the time. But that does not mean that good actors are necessarily good liars,” said Pamela Meyer, head of Calibrate, a company that offers lie-detection training.

“There’s an overlap in the skills required to do both, but they are actually separate disciplines,” she said.

The skill that most overlaps between liars and actors is convincing, appropriate emotion.

Remember how Weyrich, feigning insecurity, got so emotional she was actually crying?

Bad liars have what Meyer calls an emotion gap.

“They’re saying something really dramatic, but they’re not conveying an attitude that matches that,” she said. “Or oftentimes, we see this with in murder cases, you’ll see someone’s talking about a horrific crime, but they’re kind of devoid of emotion.”

Or maybe they convey the wrong emotion, what they think people expect, instead of what’s real.

“We associate surprise with the happy surprise party from when, you know, you had your Sweet 16 surprise party, but the average person does not like to be surprised,” Meyer said. “It’s a very unpleasant experience.”

(Notice at the next surprise party — more often than not, it’s horror that flashes for a quick moment before turning to glee.)

But pulling off pitch-perfect emotion is only one part of a good lie.

“When somebody is lying, oftentimes the cognitive load is so high on them, they’re trying to think what to say to appear composed and appear spontaneous that what they do is they leak these verbal and nonverbal indicators of deceit,” Meyer said.

The cognitive load is so great, she said, because liars have to constantly self-monitor. They have to know, for instance, that maybe they feel scared internally but have to convey sadness to their mark. They don’t have a script to rely on. Lying is basically much more work than acting.

“You can be a good actor without necessarily being able to think quickly on your feet mentally, without being mentally agile,” she said. “You can be a good actor without necessarily having an amazing working memory that allows you to hold two or three different ideas in at the same time.”

All of which sounds a bit closer to improvisation than acting. So, are improv artists the ones we should really be more worried about?

“I would agree with that,” Meyer said. “I think that a good improv artist oftentimes bears more resemblance to a good liar than a good actor does.”

Though she cautions improv can vary, some does, in fact, riff on the same types of jokes, almost like using a script.

The value of thinking on your feet, and the idea of cognitive load, made sense to Weyrich. Even though on paper she’s made more money as an actor, she said she considers herself a comedian at heart.

Her theater improv training in Sweden, where she got the forbidden tattoo, may have prepared her for the deception in Philly perfectly. There was no script, just a scene, and then off you went.

For example, Weyrich had to play a patient who is told they’d be turned into a chicken in four months.

“I’m like, ‘I don’t want to be a chicken!’” said Weyrich. “And you’re sobbing and you’re crying, and it’s so funny.”

Applying real emotion to a given situation on the fly is impressive, and pretty common offstage, Weyrich said.

In her improv-comedy circle of friends, they had to make a rule about it: No bits when you’re just hanging out offstage and off-set. Stop doing your act.

“We’ll say, ‘No bits,’ or say like, ‘Bit block,’ you know, like, ‘I’m not joking around, this is how I actually feel,’” she said.

Some of her friends are very good at it.

“I’ll have to say, ‘Dude, I can’t tell if you’re doing a bit right now or not. Should I be worried about you?’” she said.

Eventually, Weyrich dropped the insecure body image bit she had done with her mom. She came clean.

“She said, ‘Um, remember that day when I was covering up my chest?’” Polaneczky recalled. “And I said yes. And she said, ‘Um, I was covering up a tattoo,’ and I said, ‘You lied!”

At first, Polaneczky was mad. “I said, `You lied!’ And I said, `You know that that was really mean.’ She goes, ‘I’m sorry.’”

But Mom was also weirdly impressed.

“ I thought, `How did you do that?’”Polaneczky said.

Like wow, her daughter can act.

“Objectively speaking, I was like, she’s good,” Polaneczky said. “She’s really good.”