The Forgetting Sarah Marshall actor was joined by his co-star Emily Blunt for a recent interview with Collider, in which they spoke about the struggles of penning a romance.
"Yeah, it was a challenge to write, and was certainly a challenge to edit, because there are no big plot movements," Segel said.
"It's a very earnest exploration about how a relationship is fluid, and when you choose a partner and say, 'I want to marry you', you're not saying it for this moment, you're saying it for your whole life, and what's going to be ever-shifting power dynamics," he continued.
"It really is an honest exploration of relationships. It was tricky to write, but the best ones are that," Segel concluded.
Meanwhile, Blunt added that the film was enhanced by its "simple premise".
"I do think that with a much more simple premise, there's just so much more room to play amongst that," she explained. "As actors, there's a lot you can do to really make these relationships complex and interesting and messy and flawed and loving, and all of those things.
"It's a movie with great heart," Blunt insisted. "The situations these two are put in are ludicrous and outrageous and obviously heightened for the spectacle of the movie, but yet you invest in them as a couple, in a very real way, hopefully."
Blunt recently shared her advice on what makes a relationship work. She is married to The Office star John Krasinski.
Segel revealed last week that he shot a full-frontal nude scene for The Five-Year Engagement.
Watch the trailer for The Five-Year Engagement below: