With as much humility as he can muster, Sam Worthington wants to correct the notion that James Cameron's Avatar is one of the biggest hits in Hollywood history.

"It's the biggest," he says, answering a question about how it feels to be the star of one of the most successful - no, make that the most successful film in the annals of moviedom.

"You can't really fathom it," he says. "It was a changing point in my life, that's for sure. Not only getting the job and working with Jim, but everything that surrounded it. Avatar turned into an anomaly, basically. . . . When you look at the box-office figures and then try to turn those figures into how many people actually saw that film, it's beyond comprehension."

It's safe to say - and no slight intended - that The Debt, opening in theaters Wednesday, will not be the biggest hit in Hollywood history. A taut thriller about a trio of Israeli Mossad agents on the hunt for a Nazi war criminal, The Debt is also a love story and an opportunity for six actors to share three roles. Worthington, Jessica Chastain (The Tree of Life), and Marton Csokas (The Tree, with Charlotte Gainsbourg) are the young Israeli agents dispatched to East Berlin in the mid-1960s to capture "the surgeon of Treblinka," living incognito and working as an obstetrician. Ciarán Hinds plays Worthington's character, David, 30 years later. Helen Mirren is the older version of Chastain. And Tom Wilkinson has the part played by Csokas.

Directed by John Madden, The Debt toggles back and forth from one story, and time, to the other, revealing the dark secrets these characters share. And before things turn bad back there in 1966 East Germany, the three dedicated young spies played by Worthington, Chastain, and Csokas could be doing Jules and Jim - Jules and Jim with micro-cameras, codes, and guns.

"That's the thing, the triangle is great," says the Australian actor. "You get the extrovert, the introvert - my character - and the woman they're competing for. That, to me, is what the movie is about. These three people each have their own demons that they want to lay to rest, and they think this mission will help them achieve some satisfaction. Of course, it doesn't, and then we see the ripple effect that it has 30 years later."

The Debt, shot in Bucharest, Tel Aviv, and London, was adapted from a 2007 Israeli film. Matthew Vaughn (X-Men: First Class, Kick-Ass) wrote the original adaptation and was, at one point, set to direct. Then Madden (Proof, Shakespeare in Love) came along, and came to New Mexico to recruit Worthington personally.

"He flew to Albuquerque when I was doing Terminator Salvation there," says the Australian. "I thought, 'Any guy that flies to Albuquerque is a man I'll work with.' "

Worthington, 35, says that both Chastain and Csokas spent considerable time with their counterparts, Mirren and Wilkinson, to establish a continuity, and a bond, in the roles. He and Hinds, however, met for no more than 45 minutes over the course of the shoot.

"Our processes are a lot different," he explains. "I just said, 'Look, here's how I see the character, here's where his moment of change is, and here's what I'm aiming for. I don't know what you want to do with it 30 years later, mate, to be honest. That's up to you.'"

It is Worthington's - and Hinds' - character who has the hardest time coming to grips with the events of the original mission. David is plagued with guilt, with regret.

"I'm the idealist, and he's the aftermath, the broken man," Worthington says of Hinds' incarnation of the role. "It's turned him into a ghost, a shell, after 30 years of trying to rectify it. And Ciarán's got that haunted quality. . . . He nailed it on the head."

Worthington and his other Debt costar, Chastain, have a second feature together coming this fall: Texas Killing Fields, a serial murder mystery. He plays a small-town cop obsessed with the killings; she's his ex-wife.

"We're a divorced couple, with problems in the relationship," he says. "We've already done the courting [in The Debt], so you've already got the baggage there. The next time I work with Jess - and I hope there is a next time - maybe we can do the reconciliation."

If Avatar is the film Worthington is best known for, he's had a couple of other significant hits, too: Terminator Salvation, for one, and Clash of the Titans, for two.

And there will be a Clash of the Titans 2.

"We just finished, and it was one of the best experiences I've ever had on a movie," Worthington says of the sequel, reserving particular praise for his Clash 2 director, Jonathan Liebesman.

"We took what happened in the first one and said, 'Look, we don't really want it to be a continuation. It's its own beast, it's Jonathan's take on a mythological story.' "

Ralph Fiennes (Hades), Liam Neeson (Zeus), and Rosamund Pike (Andromeda) join Worthington's Perseus for the new round of titanic clashing. Look for it in the summer of 2012.

Next up for Worthington is a return trip to his homeland, to the west coast around Perth, where he grew up.

"I'm about to go do an Australian film, a movie called Drift," he reports. "It's set in the 1970s, and it's a surf film, and I've surfed all my life, so I can't complain about this one."

Worthington says he also has kept in touch with his Avatar director, whatever that guy's name is. And he says there will indeed be a sequel, or two, to the biggest movie of all time.

"Jim's told me his basic ideas for 2 and 3, and it's monumental," Worthington says. "But he's not a guy that's just going to rush into a sequel for whatever reason. He's going to do it because he wants to give the audience another experience.

"I know he's figuring things out at the moment, and when he says 'Go,' I'll go."

Contact movie critic Steven Rea at 215-854-5629 or srea@phillynews.com. Read his blog, "On Movies Online," at http://www.philly.com/philly/blogs/onmovies/.