If you could grade movie stars, Jessica Chastain would be an overachieving A student who prepares for each of her films obsessively and does way more homework than the teacher asks for.

Yet the Juilliard-trained star does not always go for the biggest and flashiest roles, sometimes opting for a smaller part in a large ensemble, as is the case in the new sci-fi thriller The Martian, which opens in Singapore tomorrow.

For next month's horror flick Crimson Peak, she also did the unthinkable and turned down the offer of the lead character in favour of a supporting part she happened to find more interesting.

And this, despite the fact that films such as Interstellar (2014), The Help (2011) and Zero Dark Thirty (2012) - which earned her a Golden Globe for Best Actress in a motion picture drama - have made her one of the most sought-after and respected actresses of her generation.

Speaking to Life at press events for The Martian and Crimson Peak, the 38-year-old redhead also reveals that she ignored advice to dye her hair blonde when she was trying to break into the industry and sometimes fantasises about playing roles that have been written for men.

But at this stage of her career, she is more than happy to cede centre stage to a fellow performer.

"I've never been the person who's like, 'Oh, I have to play the lead. Because sometimes the lead isn't the character I would be challenged the most by," she says.

With Crimson Peak, she stunned director Guillermo del Toro (Pan's Labyrinth, 2006) when she rejected his offer of the part of the protagonist, Edith, which eventually went to Australian actress Mia Wasikowska, 25. Instead, she asked to play Lucille, the character's sinister sister-in-law, in the Gothic romance-slash-ghost story.

"Everyone was shocked when they sent me the script and I said, 'Well, I'm kind of interested in this other character,'" Chastain tells Life in New York.

"And for me, it's always about an exploration. The character I play, Lucille, was an exploration in extreme loneliness and what that does to a human being.

"Maybe that's good, maybe that's bad, but I never think in terms of what it's going to do for my career or how it's going to help me."

For the record, Chastain's career is doing just fine. Her penchant for arthouse films with smaller audiences - such as director Terrence Malick's inscrutable fantasy drama The Tree Of Life (2011) and the crime flick A Most Violent Year, which many critics rated as one of the top movies of last year - has done nothing to dent her reputation as an A-list performer.

She has also enjoyed mainstream commercial success with the horror fantasy Mama (2013) and war drama Zero Dark Thirty.

More important than marketability, however, is whether she thinks she will learn something new from a project.

"I think in terms of: When I leave after making the film, am I walking away having gained something in my life? Because the film could never come out. But I don't want to spend a day of my life not growing as a person."

In The Martian, she plays the leader of a team of astronauts who must figure out a way to help one of their group who was left behind on Mars. But although the central character is the stranded astronaut, played by Matt Damon, Chastain felt like she "grew a lot" playing the supporting role.

Apart from getting a chance to work with legendary director Ridley Scott (Blade Runner, 1982), it meant she would get to train like an astronaut at the National Aeronautics and Space Administration's Jet Propulsion Laboratory.

"When I first met Ridley, he asked if I had any questions and I said, 'Yes. If I do the film, can I go to space camp?' I used it as an opportunity to fulfil this dream, I knew many doors would be opened - I'd be able to meet astronauts, go to the laboratory and meet the scientists and see the work they're doing, and do virtual reality with the images coming in from Mars. It was pretty far out."

One of the real-life astronauts she met was Tracy Caldwell Dyson who, despite Chastain's protests that she would make "a terrible astronaut", reveals that the actress possesses many traits that would serve her well in outer space.

"She's a good student, she does her homework," says the 46-yearold. "And I'm not just talking about this film. If you watch her in any film, you can see she's a believable character because she gets her work done. And that's what an astronaut does."

With Crimson Peak, this studiousness impressed someone just as meticulous - director and horror maestro del Toro, who assigned each actor a lengthy reading and film list to help them prepare.

Wasikowska was tasked with reading Gothic horror classics such as Mary Shelley's Frankenstein, but Chastain was already ahead of the game, having already devoured that and the work of Henry James.

Then she delved into psychology texts and the work of the 18thcentury "graveyard poets" - all because the director's backstory for her character suggested she might love these poems, even though that detail would never make it into film. And she spent much of her free time perfecting a classical piano piece because her character plays it for a few brief moments.

It is this sort of dedication that has led to Chastain - who popped up on Hollywood's radar when seven of her films were released in 2011 - openly declaring herself as a workaholic.

"Whenever I'm on a set, I try to use my downtime to keep me in the energy of what the movie is. So for Crimson Peak, I watched (the television crime show) Dexter. I just filmed the Snow White And The Huntsman sequel, so for the first two months, it was reading and watching romances.

"There were two years that I didn't have a vacation, but you know, I feel like a workaholic who's never worked a day in her life. I have the best job in the world because I love learning about people and the psychology of characters, so for me, being on set is the most wonderful thing."

Which is not to say she would not change anything about her job or industry. The actress - who helped produce her 2014 drama The Disappearance Of Eleanor Rigby, an experimental three-film project - says she hopes to use her growing clout to get more films made "that have diversity in them".

"When I attach myself to a film now, the great thing is I can talk to the director and say, 'What do you think about a woman playing this role?' or 'Wouldn't it be great if we could get a Latino to play this part?' To try and create a realistic version of the world. Because I think Hollywood lacks in that sometimes."

Echoing actor-director George Clooney's recent suggestion that more male roles be rewritten for women, she says: "Just change it to a woman. We're not that different. My character Murph, in Interstellar, was originally part of a story about a father and a son, but what makes it so interesting is that it became about a father and daughter because that's not a story that's been told over and over again."

She admits there is a part she recently had her eye on that was written for a man, but diplomatically declines to reveal which it is: "There was a role that I thought would be really interesting if the genders were reversed, but they didn't do it."

As an actress, she admits that she has been lucky ("I'm getting really great roles").

"But as an audience member, I've been disappointed by the lack of female protagonists. This year I'm excited, there are a lot more stories, but we've got to see how long this lasts."

The Martian opens in Singapore tomorrow and Crimson Peak opens on Oct 15.