Opening night at the Tribeca Film Festival, and Jason Segel is there with The Five-Year Engagement, the romantic comedy (with serious stuff) that he stars in with Emily Blunt — and that he wrote with his Forgetting Sarah Marshall partner, Nicholas Stoller. It's a big deal, kicking off one of the premier festivals in the land, and Segel was at the ready, introducing his film to the betuxed and begowned with Henny Youngman-esque elan.

"Marriage is a three-ring circus: engagement ring, wedding ring, suffering," he quipped, and then let the crowd settle in to watch his character and Blunt's go through the throes of dread, doubt, break-ups, and makeups that are his film.

And afterward, there was Robert De Niro, the actor god and cofounder of the Tribeca fest, just standing there, milling around with De Niro-esque elan.

"I talked to him very briefly," Segel says. "But I was so starstruck I knew I wouldn't be able to maintain a proper conversation, so I just said my hello and moved on. ... Still, it was incredibly cool."

Although the couple Segel and Blunt play in The Five-Year Engagement are happily living together and clearly in the relationship for the long run, once he proposes to her — in a sweetly flubbed scene on the deck of a San Francisco restaurant where Segel's Tom Solomon works as a sous chef — things start to go wrong. Career opportunities intercede, her sister and his best friend announce their engagement, one delay follows another. The strains begin to show. The film, directed by Stoller, and boasting a great cast of supporting players (Chris Pratt, Alison Brie, Lauren Weedman, Jacki Weaver, Rhys Ifans, Dakota Johnson, Brian Posehn), opened in theaters Friday.

"The rules of engagement have changed a little bit, no pun intended," says Segel, in New York and on the phone from the lobby of his hotel, reflecting on how the nuclear family concept isn't what it used to be. "Nowadays, more than even just a generation ago, you can be a single parent, or you can have a perfectly happy unmarried household, a partnership." But while there may be less trepidation about the idea of marriage, Segel suggests, it is still a daunting thing, and that's what he and Stoller wanted to explore.

"We wanted to hark back to the great romantic comedies, like Annie Hall and When Harry Met Sally," Segel explains. "They're uproariously funny — and I like to think ours is as well — but they also deal with the real issues of a couple trying to figure it all out. …

"With a lot of romantic comedies, these ridiculous obstacles are set up for no good reason. 'He's a scientist, but she hates science!' And that's not what a real relationship is about. This is just about a normal couple dealing with the issues of how fluid and hard the dynamics are over a long stretch of time. And one of those things when you say 'I do' is to realize that 'I do' means I'm choosing you as a partner for what is sure to be a very rocky road. And you're the person that I want to do it with."

Segel is not a procrastinator. He's in the cast of CBS's How I Met Your Mother, he starred in (and cowrote) last year's Muppets reboot The Muppets, and he has the title role in the Duplass brothers' Jeff, Who Lives at Home. When he and Stoller sat down to write The Five-Year Engagement, they already knew they wanted Blunt for the role of Violet, a serious-minded transplanted Brit keen on pursuing a career in social psychology, who meets Segel's Tom at a costume party. He's in a pink bunny suit, she's dressed as Princess Di. Segel has known Blunt for quite a few years now, is good pals with her husband, The Office'sJohn Krasinski, and worked with her on theJack Black vehicle Gulliver's Travels.

"One of my big pet peeves about romantic comedies is how very often they just cast two viable Hollywood actors, who both had successful movies the year before," he says. "They don't know each other, and when it gets to the moment in the film — which good romantic comedies should have — where the audience is asking, do we even want them to stay together … Well, I always feel, when I watch those movies, like I don't give a damn. And I don't think the actors do either!

"Whereas, in this, Emily and I clearly have a rapport. We've been friends for a long time, and when we get to that moment in our movie, I think you're rooting for them for just that reason. They seem like best friends. …

"Obviously, we both know how to act," he adds, and not in a full-of-himself kind of way (really). "So the big plot-movement scenes, we're going to be acting our asses off. But it's the moments when we're just walking down the street together, and Nick, our director, says, 'I just need a shot of you guys walking and talking, so talk about whatever you want, it's just a filler shot,' and we're making each other laugh and enjoying each other's company — those scenes are the glue that makes the whole thing stay connected, I think."

There's a big fight scene in the middle of The Five-Year Engagement — the pair transplanted from the beautiful Bay Area to snowbound Ann Arbor, where she has landed in the psych department at the University of Michigan.

"We worked really hard on that scene," Segel recalls. "One of the things we're proudest of is that that is how people argue: It's not perfectly worded, it's clumsy and awkward, and those are the kinds of scenes that you often see in movies and people are just being incredibly articulate about their feelings. And I always think, if you're able to be that articulate and composed, you wouldn't be fighting. When you're fighting, the worst of you comes out. Things like 'Your face makes me stupid!' It makes no sense, but it's someone struggling to try to express what's going on."

Segel will be struggling to express what's going on come this summer, when he plans to write two screenplays before returning to the set of How I Met Your Mother in August.

"I've been telling people that I'll be taking the summer off for the first time in seven years," says the industrious TV and film actor and scribe. "And that is my plan. I'll be writing a couple of movies to potentially shoot over my next hiatus. … That's what I call taking the summer off."

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