Kazan and Radcliffe's characters meet at a party. They talk. They understand each other.
And then what?
On one level, What If (it opened Friday at the Ritz Five and Carmike at the Ritz Center/NJ) examines male-female relationships, what makes them work, or not.
"I tend to think it's a question for which there is no answer," says Kazan, on the phone from New York the other day. "It's a subjective and complicated issue. And it's pretty case-specific. I have a lot of male friends, and I don't have any trouble not sleeping with them.
"And I think they're attractive," she adds, with a laugh.
"People get into trouble because they want to get into trouble. And I do think it's hard to navigate attraction. . . . And that's a separate issue from friendship."
In What If, which Kazan, Radcliffe, and company (very good company: Adam Driver, Mackenzie Davis, Megan Park) shot two summers ago, friendship is key.
"One of my favorite things about the movie is actually how good these two people's friendship is," Kazan says. "That even though there's this undercurrent of desire, actually one of the reasons that they're hesitant to treat each other as romantic partners is because they're so good to each other, and good with each other, as friends."
In fact, the original title of the film was The F Word, and not the F word you're thinking of.
"It was very much F for 'friends' - they were being a little cheeky," Kazan explains. "But basically they wanted to get a PG-13 rating, so they cut a little language out, and the MPAA didn't like our title, so we changed it."
Kazan had met Radcliffe only once, fleetingly, before shooting. The concern that they wouldn't be able to generate heat in front of the cameras, she says, "has to be in your mind somewhere."
The worry was quickly put to rest.
"The truth is, I've played opposite people that I had to be romantic with many, many times now, onstage and on film, and I've never had a problem finding something appealing and attractive about the other person.
"Sometimes you can't help that lack of chemistry - it's just a weird mix, and we've all seen movies like that. But most of the time a lack of chemistry is actually indicative of a lack of curiosity between the actors.
"And Daniel is somebody who's seeking to be connected as an actor. And so am I, so I wasn't really worried. Although we did have a lot of people crowing over the chemistry that they felt that they were seeing onscreen - so any doubts that I had were quickly allayed."
Kazan, 30, is the daughter of screenwriters Nicholas Kazan and Robin Swicord. Her paternal grandfather was director Elia Kazan. She's partners with Paul Dano, the There Will Be Blood actor. The two teamed on the 2012 romantic comedy Ruby Sparks, which she also wrote.
She has written for the theater, too ( Absalom, We Live Here), and acted extensively onstage. Sunday, in fact, marks the final performance of When We Were Young and Unafraid, the Off-Broadway production of Sarah Treem's 1970s period drama. It opened in May.
"It's about a woman, played by Cherry Jones, who runs a safe house in Washington state for battered women. And I play someone who comes looking for asylum."
Reached a week before the play's closing, Kazan said she's going to miss the experience.
"It's so much mixed feelings. I mean, mostly it's just sadness. I am going to find it so hard letting go. . . . But I'm also relieved. The play is very difficult, emotionally. There's been a lot of crying for me this summer."
In a few weeks, Olive Kitteridge - an HBO mini-series adapted from Elizabeth Strout's 2009 Pulitzer Prize-winning novel, a series of interlinked stories set in a coastal Maine town - gets its world premiere at the Venice Film Festival. Kazan stars opposite Richard Jenkins, Frances McDormand, Bill Murray, and Martha Wainwright. Lisa Cholodenko ( The Kids Are All Right) directed. It's set to be shown in November.
"I had a wonderful time doing it," Kazan says. "It's really, really good."