“You know, it’s a real funny, ironic thing. I am the happiest person; literally, I am annoyingly happy to some people,” Shailene Woodley laughs. “I was naturally born an optimist, I love life, even when things are not going so great. But for some reason, I guess I can tap into my dark alter-ego emotional side very easily [laughs] and play these characters on the screen.”

Putting aside any accusations of irksomeness, it’s far too early on a Sunday morning — especially one following a late Saturday night spent at a party thrown in her film’s honor — for Woodley to fake this kind of energy and excitement. “Maybe that’s why I love acting subconsciously, I store all my angst and let it out,” she continues, shrugging her shoulders, either modest or bewildered by her growing career as a dramatic actress.

Whether she’s tapping into something deeper or just really good at pretending, the 19 year old actress is doing something right. Already playing a teenage mother in the hit ABC Family series, “The Secret Life of the American Teenager,” Woodley is set to break into the larger national consciousness with a star-making performance as Alex, George Clooney’s foul mouthed, drug-abusing daughter in “The Descendants,” which has so far been the critical darling of the fall festival circuit.

From the moment early in the first act in which she’s brought back home from rehab to help deal with the fallout from her critically injured mom’s impending death, Woodley dominates screen time and launches into the film’s most profound transformation. Drunk and hitting golf balls into the ocean when first picked up at her rehabilitation facility, a newfound familial responsibility casts her alongside Clooney in the story of a father and daughter forced to grow up together, side-by-side.

“I think a 17-year old going through the most angsty periods in her life, thought the world was out to get her, kind of took on the victim thing, kinda said ‘f*ck life,’ and through this movie and through this beautiful arc, she has a chance to not only reconnect with her family but reconnect with herself,” Woodley says, reflecting on the fondness she felt for her character.

“I think she created such a barrier, such a wall around her that she never was really forced to examine her life, and the consequences of her relationships,” she continues. “And in the end, she’s become vulnerable — and still independent, but independently co-dependent.”

Early after her return home — to a small island in Hawaii, much of which is owned by her dynastic family — Woodley’s Alex sets the film’s main course of action by divulging a secret that hangs an even darker cloud over her coma-stricken mother’s tragedy. Instead of just a speedboat accident victim now wilting away as her life support is removed, Alex divulges to her dad that his wife was sleeping with another man, caught in the act while Alex roamed the neighborhood during her school’s winter break. Once she tells her father the truth, the pair — along with her younger sister — set off to find the man with whom the dying woman slept.

It would figure that to tell her father such a harsh truth, instead of allowing him to more fondly remember his dying wife, would be a difficult decision; instead, in this case, Woodley says the character’s penchant for mature past times and beverage choices belied her maturity and ability to keep the secret.

“It’s a giant decision. I don’t think she would have — I think she saw him and saw how lost he was, and then the thought of losing her mom, that’s a giant secret to hold, y’know?” the actress asks rhetorically, showing sympathy for her Alex’s dilemma. “I think she saw it as, I don’t want this secret, I want to get this out. Yeah, she’s 17, but she’s still a little girl in terms of the world in relativity, so I think she needed to get that secret off her chest, and I think she saw her dad as a really weak person, and she wanted him to know that this is what is happening, you can’t be in denial, you can’t be stupid anymore, you need to grow up, dad.”

Her forcefulness on screen, though, belies a casual humility off-set. She offers nothing but praise for her co-star Clooney, and insists that any certain delivery came at the suggestion of the film’s writer-director, Alexander Payne.

“I didn’t know if we were — truly — I didn’t know if we were making a comedy or a drama,” Woodley admits, deferring credit, for the umpteenth time, to the filmmaker. “Because, we would do one scene one way and then do it another way. And it was just Alexander going ‘Okay now instead of glaring, why don’t you smile? And you’re like, ‘What? That makes no sense! But I trust you,’ and you do it, and after you do it, you’re like, oh, I see what he was thinking.”

She may sound as if she’s uncertain about her craft and talent, but for a 19 year old, her career hasn’t exactly fluttered along on mindlessly happy, Disney Channel-type subject matter. While it often takes actors and actresses years to get the opportunities to tackle more serious fare, Woodley’s work has had a greater weight; death and estrangement in “The Descendants,” and teenage sexuality in “The Secret Life.”

“The most positive compliment I’ve ever gotten,” she remembers, “was from a mother who came up to me and said, ‘Thank you and your writers and the rest of the cast for this show, because it has opened up conversation outlets within our home, I can talk to my daughter and son about sex now, and I can talk to them about the consequences of it, and what you need to know before you make that decision and after that decision,’ and she’s like, I would have never talked about that with my teenagers. And I think that’s where I think a lot of people get lost in teenage sexuality, is because they’re uneducated about it.”

Having served as that vessel, it’s fitting and fateful that her first major film role comes as she exits her teen years. Time to grow up not just on screen, but in the characters she gets to play, as well.

“I’ll be 20 in a month and I’ve always played high school characters,” she says. “I think it would be fun to do more of a college setting or a young adult setting, versus a teenage atmosphere. And I’d love to do a comedy, I haven’t done a comedy thus far, so I think that would be great.”

Then again, dramas aren’t so bad either — and she’s got high ambitions for those, too, name dropping Natalie Portman’s turn in “Black Swan” as her ideal role. For the moment, though, she’s letting herself take in all the madness that comes with starring in a hit, awards season shoe-in film for the first time.

“I always say that the audition was the cake, the performance in Hawaii was the icing on the cake, and this, reconvening with George and Alexander and Fox Searchlight and all these beautiful people, I don’t even know what this is like — 20 jars of maraschino cherries or something,” Woodley laughs. “So I’m just taking it all in stride and waking up every day with a smile on my face and in gratitude for what that day holds. And I have zero expectations, whatever happens, happens. It’s beautiful either way, so I’m enjoying every moment and so grateful for every moment.”