Since having her breakout moment four years ago, Jessica Chastain has established herself as Hollywood’s ideal of smart beauty. The Oscar-nominated star of “The Help” and “Zero Dark Thirty,” now onscreen as the ’80s-glam, viperish wife of a heating-oil businessman (Oscar Isaac) in “A Most Violent Year,” is renowned as a workhorse (she appeared in six movies in 2011 and four in 2014). But as the actress reveals, that wasn’t always the case.