After the memorable ending of last night’s Parks and Recreation(which I reviewed here), I emailed co-creator Mike Schur a few questions about why they chose to do it, and what it means for the show going forward.
 
Note: the very last answer is somewhat spoiler-y about the premise of the next episode, but it’s also Schur clarifying something that is very strongly implied at the end of this one.
 
When and why did you decide to do this? Why is this the right time in the life of both the show and of this relationship for Leslie and Ben to get engaged?

At the end of season four we decided to move Ben to Washington for a chunk of episodes, and then we began to discuss reasons he would come back.  We wanted him to succeed at the job -- he's a pretty capable guy -- so we thought it was unreasonable for him to say, "Well, that was fun, running a Congressional campaign. Now it's back to a much smaller job in Pawnee, Indiana!"  Him realizing that his relationship with Leslie is the most important thing in his life made a lot of sense -- the proposal was the simplest and cleanest (and most romantic) way for him to declare it.
 
How did you feel those episodes worked out where Ben and April were in Washington? Was it easier or harder than you expected to have two of your regulars existing largely independently from the rest of the cast (including both their romantic partners)?

When we committed to shipping Ben out for five episodes, we wanted to send someone with him, and April seemed like a fun choice.  Both because it fits her very slow arc of maturing a little and trying to achieve more in her life, and because they are two characters who haven't had a ton of scenes together.  I loved them together.  It was like a little show within a show: Ben Wyatt and his terrifying little sister, who begrudgingly came to like him. And it wasn't that hard -- I had practice from Season Three of The Office, moving Jim to Stamford, and we regularly have three or even four stories in an episode where the characters are divvied up. 
 
How, if at all, does this couple, and each half of it, change as a result of the engagement? Is Leslie Knope with a ring on her finger appreciably different from the awesome version we already know? And what have you learned from several years of writing for Mr. and Mrs. Andy Dwyer about making happy couples funny and interesting, contrary to the conventional wisdom of your business?
Leslie's life has always been a balance of the personal and professional, and she takes both very seriously.  And Ben is a pretty self-possessed guy, who's centered enough to not only handle but actually prefer someone with the drive and ambition of Leslie Knope.  Neither person is going to change or go spinning off the rails in any way.  The only difference is that their lives are officially joined now.

As for how to keep them funny and interesting: as long as we keep coming up with good stories for them, and take their lives and careers in new directions, that will happen naturally.  Because the characters are played by Adam Scott and Amy Poehler.  Who are good at acting and comedy.
 
Given where the show stands right now, and the uncertainty about most NBC comedies, was this in any way done with the thought that you'd better do it now, just in case?
 
We've had that attitude, frankly, since the end of Season 2.  Go for broke.  It's scary, because things advance at (what for TV is) a breakneck speed, but on the other hand, interesting stuff happens!
 
Does the proposal mean he's not going to go run Jack Scalia's campaign for governor? Or is this his way of assuring Leslie that he's in it for the long haul even as he's traveling the country?

He will not work on the campaign in Florida.  Episode 7 involves him contemplating a new job in Pawnee.