I can see you and I are going to be at polar ends of this argument.  Perhaps one of the reasons the question "What's my motivation?" has become such an actor-centric cliche is simply because it is a question that the actor must ask CONSTANTLY (preferably, of himself and not the director).  It is the actor's JOB to find motivation for everything he does.  Like it or not, by the time opening night rolls around and there's an actor onstage who's STILL unsure what they're doing, then they have no business being on stage.

Even Shakespeare (no matter what version you're counting) wrote many things that are largely expositional, plot-driven and descriptive due to the limitations he faced, yet rather than have a narrator step forth and do it (as he did in the prologue of "Romeo & Juliet") Nine times out of Ten he CHOSE to have a character reveal the information.  As an actor playing this character, it suddenly befalls upon me to understand WHY I'm stopping the action of the play to disperse all this.  And as an actor LISTENING to this speech, it's up to me to figure out why I'm standing there, taking all this in and not saying "Yes, I can see that, you dolt!"

Sometimes certain information is difficult to work into natural conversation and so the playwright leaves it out.  Sometimes, it might be an idea that the playwright hadn't thought of yet helps an actor discover his motivation.

As I'm paraphrasing what was written in a previous post on another thread "The parts in black [ink] belong exclusively to the playwright.  The white bits are mine to do with as I please."

Writing character bios CAN be a wasteful exercise -- but that's simply all it was -- "an exercise."  We never changed a single word of dialogue or altered any phrasing of the script to suit our bios.  What WAS altered was our attitudes toward the characters and what they were saying.  We began to see them in a new light, with more depth.

On the contrary!  If you're devoting that much time to discover the nuances of EVERYTHING, then you begin to respect the script even more!  This was the lesson I learned (and I was trying to impress on the others on this forum).  Our initial reaction to the script was that it was a worthless piece of fluff.  The director didn't want that attitude to come across in the production.  She FORCED us to discover the nuance, depth and meaning which made me realize:  Who am I to decide which script is worthy of my respect?  Treating EVERY script as if it were brilliant makes doing the job of an actor worthwhile and often challenging.

Playwrights might know that, but not always.  And if they do, then they're relying on the actors to bring their own personality or ideosyncracies to the performance to flesh it out. 

As someone who's also written scripts, whenever I (unintentionally) saddle a poor actor with an under-developed role, I become absolutely THRILLED when the actor finds ways to breathe life into it.  First of all, it makes me look good -- like I knew what I was doing in the first place. 

As far as I'm concerned a character can never be "too three-dimensional (?!)"  It's precisely that goal that I attempt to achieve as an actor.  It's what I look for as a director.  And it's something I greatly appreciate as an audience member.

Whenever small children undertake any kind of production, it is often the adults who drill into them "Learn your lines and say them loudly."

Unfortunately, for many actors, this is the ONLY lesson they've learned from acting and never waver from this.  What I'm trying to say is there's more information BETWEEN the lines, if you only look for it. 

How is this different from a designer deciding exactly what shade to paint the set, or a costumer selecting what type of collar should be on the leading lady's blouse?  If it's not specified in the script, (and unless the playwright is sitting in on the rehearsals) then we can never be absolutely certain what the playwright's intentions were. 

I agree, some forms of theater don't lend themselves to this philosophy.  But I disagree about "farce" being one of them.  Nothing spurs characters on to more and more outrageous situations than finding the reasons for their actions in farce.

And for an actor to dismiss motivation altogether as "irrelevant" is an attitude that I find reckless at best and arrogant at worst.  I've seen too many actors rely simply on the words the carry their performances (especially in our old friend, Neil Simon).  They figure the lines are smart enough or clever enough to make up for any characterizations they might neglect to create.

When Mike Nichols directed the first production of Simon's "Barefoot in the Park" he told the cast at one point to "forget we're doing a comedy and let's pretend we're doing 'King Lear.'"

He knew the value of investing importance in every line.  Motivation is what makes acting real.  It makes characters believable.