Martin Scorsese’s devilishly convoluted cops-and-mobsters saga “The Departed” presents Leonardo DiCaprio with one of his most complex acting assignments to date: He’s Billy Costigan, a lean, bristling rookie cop assigned to infiltrate Boston’s Irish-American mob, and then required by the mob to help them root out the mole they sense is in their midst.“The initial challenge, on reading the script, was to ask myself as an actor what I can emote to an audience when I’m surrounded by people who are trying to determine my identity, and I can’t reveal much of what I’m going through as a character,” DiCaprio acknowledges. “I found I didn’t need to do much, because the tension around me in this dangerous underworld got people to tap into that on their own.” While he had tackled dual role-playing before in “Catch Me if You Can,” DiCaprio points out, “I’ve never done anything as tension-filled as this movie is from start to finish.” Because of his real-life friendship with Jack Nicholson, who plays the flamboyantly depraved mob boss Frank Costello, DiCaprio says, “I had a hard time at first understanding the dangerous circumstances my character is in.” That changed with some inspired improvisation by Nicholson, who rattled his co-star by going off-script to light a table on fire and thrust a gun into his face while demanding that he explain himself. “You can’t help but have your adrenaline come out as an actor in a situation like that,” DiCaprio says. “It was one of the more memorable moments I’ve had, definitely.” Even playing across from Matt Damon, whose criminal-infiltrating-the-cops role is the mirror opposite of his, to the point that the characters dress almost identically and pursue the same woman, DiCaprio finds a way to make Costigan’s anguish over the position he’s in searing and distinctive. “DiCaprio does himself proud in a risky role that stabs at the heart as Billy’s bravado loses the battle to his jangling nerves,” applauds Entertainment Weekly’s Lisa Schwarzbaum. The dual role is “the best performance of his career,” raves Richard Roeper of the Chicago Sun-Times. Between “The Departed” and “Blood Diamond,” much has been made in the press of the idea that the former teen star, now 32, is choosing hyper-masculine roles and growing up onscreen. “Moviegoers used to his eternal boyishness will be pleasantly surprised at this, his first fully adult role, and the genuine substance and maturity he brings to this part,” says the L.A. Times’ Kenneth Turan about DiCaprio’s role in “The Departed.” The actor finds the idea amusing. “In terms of choosing a certain role because it would make me seem more manlike?” he laughs. “I’ve never thought about that, ever in my career.” Next project: “The Eleventh Hour,” a feature-length doc he’s producing, co-writing and narrating, examining the state of the global environment.