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. 

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