There are several equally important considerations to juggle:
1) Sturdiness - A chair or sofa onset has to survive. Depending on the script and blocking, an actor may jump on that sofa, commit murder in that rocking chair, or dance on that coffee table. You need good communication between director and designers so everyone knows what this furniture needs to do.
2) Shape and Size - Maybe an actor needs to hide behind this sofa, so you want a high back. Or maybe it's imperative the sofa not hide important business behind it, so it needs a low back. Know which! Likewise, you may need a round table to allow flexible seating around it or a very small one because space is cramped. For story-telling reasons you may want an extra big and pompous desk... so big and pompous you need to build it.
3) Style and Color - The "look" of furniture onstage is important, since it will reinforce (or undermine) the scenic design. Hard to pretend this palace is in the 18th century if that chair screams 1978's avocado green! (Did Louis XIV even have an over-stuffed corduroy Lazy-Boy recliner back then?) One vital question when borrowing is always, "Can I paint or reupholster this?"
4) Availability - Is-it-possible-ness includes both the ability to borrow and the budget to buy, build, or reupholster. This is the problem with happily sketching The Perfect Sofa into your scenic design. The director will love it... but you'll never ever find it.
For this play we looked first through Kitchen Dog's stock of furniture and photos of other pieces from the homes of Dogs, but nothing was quite right as the "hero" rocking chair of The Beauty Queen of Leenane
So yesterday the director, prop designer, and I visited another theater's warehouse to pull furniture. Among the pickup truck plus one small SUV and a car's worth of stuff we picked, we also debated which of three wood rocking chairs would best suit the show: one was Colonial (of the gracefully curved head and dowel-back type) in faux mahogany; another was more rustic and straight-lined with a "pickled" finish, but with a too Texas-porch-looking slatted seat; and the third was a classic blond Bentwood rocker. All were sturdy.
I nixed the Bentwood immediately as too '60s or '70s looking. The director and I preferred the weathered look of the pickled finish over the gloss of the mahogany, as well as the pickled chair's plainer lines.
But what about the wrong-looking slatted seat?
The Mother-character who sits in it is an invalid... So I'll pad the seat to fit the character, comfort the actress, and hide those slats. Multiple cushions maybe? Going for that layers o' stained fabric cat-bed kind of look? I was lucky enough to find a sheepskin. Bed-ridden patients lay on sheepskins to avoid bedsores, so why not sit on one? Perfect!
That's how furniture is chosen.
Now multiply that same discussion x however many pieces of furniture needed. With the added consideration (which any interior designer would understand) of calculating how well each piece of furniture chosen will interact design-wise with all the others!
Public domain image from Wikimedia Commons
A note on that photo of a rocking chair: I found it illustrating a funny and true rant, "Stuff That Doesn't Completely Suck Part I," that discusses worthless versus sturdy chairs HERE at Gordie's Lounge.
My own rants on furniture? "Return of the Dreaded Sofa" HERE.