Ryan Reynolds and Isla Fisher in Definitely, Maybe Andrew Schwartz/Courtesy of Universal

When a young and idealistic Will Hayes arrives in New York from Madison, Wisconsin, one of the first people he encounters is Summer Hartley (Weisz), a seductive journalism student who is having an affair with her professor (Kevin Kline). "We knew that the part of Summer was someone larger than life," says Brooks. "And we wanted an actress that had that quality and already had that kind of relationship with the audience. So if we were going to have a bigger movie star in [the film], Summer Hartley was always going to be that part." Friendship between Will and Summer turns to romance but when her ambitions as a journalist overshadow her relationship, the two part ways. "She is definitely a free spirit," notes Weisz of her character. "She is after a good story. I don't think her politics are what matters. She is a true journalist. She is after the good story."

Will begins working in the presidential campaign offices for Bill Clinton as the coffee guy and office gofer. There he meets April. She is there simply to earn money and doesn't appear to give a damn which candidate wins the election. For the part, Brooks met with Isla Fisher (perhaps best known for her turn as Vince Vaughn's unhinged stalker in Wedding Crashers). "April is a character who is on the one hand a downtown New Yorker but we didn't want that to be a cliché. And we wanted someone that had this incredible inner life and spunk — but at the same time didn't feel sort of obvious. Isla just has this incredible life inside of her. And she has brought a lot of hyper-kinetic energy to the part, and that has been great." For Fisher, the part of April was an interesting challenge because the role demanded that she and Reynolds develop a combative relationship that fluctuates between true friendship and sexual tension: "She is the friend in the story but there are kinds of currents going on between the two of them [and] the wires are crossed. There is a lot of miscommunication. I think that their relationship begins with a sexual attraction but they are at the wrong place in their lives to pursue it at that juncture. But it develops through their friendship over time."

And then there is Emily, Will's college sweetheart. "For Elizabeth Banks, who plays Emily," says Brooks, "what was needed was someone you believe is an eighteen year-old co-ed who falls in love as a freshman in college with Ryan" but who then who could also be a part of his complex life over ten years later.

Abigail Breslin and Ryan Reynolds in Definitely, Maybe Andrew Schwartz/Courtesy of Universal

But perhaps the greatest love of Will's life is his daughter, Maya, played by Abigail Breslin (Little Miss Sunshine). Acting opposite an actress not yet in her teens, says Reynolds, "definitely comes with a few challenges" but he says he found the key lay in just relaxing and letting the relationship grow. He recalls taking Breslin to a museum and spending time in a couple of toy stores before the shoot so that the two had time to forge a friendship off-screen: "The thing with kids is… you can't force anything; you have to let it happen naturally. So, the hardest part for me and Abigail was to allow our relationship to be whatever it was gonna be. And not try to push that or force some sort of false intimacy."

"Sometimes she seems to be the most professional of all," laughs Brooks. "For a young girl, she has enormous unbelievable ability to focus, to listen, to be listening to the other actress when she is not on camera. She is quite extraordinary that way. It is amazing how we don't lose any time at all because she's a kid. She seems to have a kind of inherent emotional understanding about what is going on in the scene."