Strangely enough, they don't usually say much more to me about sets for their shows than the dead ones do.
This is partly because the playwright writes into their script whatever they feel strongly about as a scenic element - and then whatever doors or furniture the writer envisions are required or implied by the play's action. The "look" and the details for any particular production are then left to the director's and designer's discretion.
Even when a living playwright is in rehearsals and actively rewriting the play, they seldom make much comment on the proposed set as it's being designed and discussed. The set is still in the director's purview. The etiquette, I suppose, is that if the playwright really hates something scenic, they'll have a quiet word with the director, who might pass on the criticism to the designer.
Generally the only opinion I hear from the playwright is praise at the Opening... Mostly this has sounded sincere, I think. Once in a while it's clear that the playwright is genuinely thrilled by a production and maybe the set too: that's wonderful.
Kitchen Dog Theater - where I'm a company member - specializes in new works, so I've been especially fortunate to work on world premiers, to sit in on read-throughs and rehearsals, and to talk with several real live playwrights! Right now I'm working on the new play Ruth, whose playwright is also a company member. It's fascinating to hear back-story and author's reasoning or intent for things in the script.
The occasionally difficult part of having a living playwright - from the designer's point of view - can be that sometimes, in modifying this work-in-progress-play, the writer will change or add locations.
Savvy playwrights have a good notion of what's feasible to put on stage (at the likely budget) and they'd never add a new, scenicly demanding location late in the process, but I was once startled by one who told the theater company the script had changed (they didn't realize it was being revised!) and a big scene had been added: a character spontaneously combusting in a brand-new-to-us outhouse!
My whole set (a complete house with swamp) had to shift stage left in a hurry to make room for this exploding outhouse.
It was a cool effect though.
Color sketch for the set at WaterTower Theater
See the stand of reeds on the far left of the drawing? (Stage right.) That's where the outhouse ended up.
The actress (the "bad" sister) went into the rustic outhouse; spontaneously combusted, with flame-colored light and smoke leaking from the plank door; came out again, now looking ashy and wearing a singed feather boa; met the Devil (in a nice suit) on a little bridge over the swamp, whose water lit redly and bubbled as if boiling (a very cool effect); and was led away, protesting comically.
Funny stuff. I didn't mind moving the set for it.
(How'd we "boil" the swamp? Easy. Red pool lights and a garden soaker hose - the kind with tiny holes in it - through which air was pumped.)