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. 

Answer the tuba 4;