I have gotten the book Being Hal Ashby (life of a Hollywood Rebel)as it was mentioned here by Jason.Thank you.

Going first to the chapter Being There,I have some quotes to post,esp for those not yet able to get the book.

Some tid-bits. Burt Lancaster was first thought for the role of Ben Rand. But "for the role of the U.S. president,Ashby could not think past Jack Warden,delighted to be working with him again since Shampoo".

Most details we know such as : Once Sellers found the Chance character through voice and walk "he retreated inside the persona which resonated profoundly with his own sense of self ". 

"Sellers described the experience of working on Being There as "so humbling and so powerful"

"He would sometimes call CUT in the middle of a scene when overwhelmed. Asked  if  he was all right he'd say ' Oh. No,no I've just never seen anything quite like this film before'

He would say it in the voice of Chance.

To remain in character he would often retreat into his trailor. "He would only return to himself when relaxing with Jack Warden .Sometimes laughing until they cried with laughter."

Ashby was "in awe of Sellers and let the actor's rhythm set the film's pace."

One quote I feel says a lot ,at least to me. 

"Just after Sellers's death Ashby recalled ' I always felt while looking at his eyes how I'd never talked with anyone who was so totally vulnerable. He always let me into him through his eyes to places so deep that I wouldn't ever try to know. Places that only he,if he were aware of it at all,could know about."

 

It's hard to get past that quote.I read it over and over.

 But...."on the last shot of Being There,after Sellers final scene,the crew clapped,whistled,and cheered for 15 minutes,while Sellers and Ashby stood teary eyed. Ashby's admiration for Sellers had grown during filming and he maintained ' That Being There proves to a whole generation that the man's a F--ing Genius !  Peter Sellers has every right to sit up there on his mountain in Switzerland and SCREAM ! I'm all right Jack ! Screw you ALL !  "

 

(me) ......   Right and Double Right-for "generations" and FOREVER.