Who maintained that the actor should be coldly unemotional? “[a] great actor . . must [be] an unmoved and disinterested onlooker. . they say an actor is all the better for being excited, for being angry. i deny it. . actors who play from the heart . . are alternately strong and feeble, fiery and cold.”

The person who maintained that the actor should be coldly unemotional was Denis Diderot.
Denis Diderot was a French critic, philosopher, and writer, and he wrote in his "Paradox of the Actor" what a perfect actor would behave like. In his opinion, no emotions should be present when that person acts, but rather the actor should show rational intelligence and aesthetic judgment. 

The linear perspective changed the quality of art because it gave art a sense of reality. it gave the illusion that the background and surroundings were actually close and/or far and a sense of depth. linear perspective change the quality of art following its birth the renaissance period as they want the art to be more realistic and they believe one way to achieve it is to use the method of linear perspective. it uses the principle of math to realistically portray space and depth in art.