The stage director of the first Porgy and Bess production recalls hearing the score in Gerswhin's New York apartment:
They both blissfully closed their eyes before they continued with the lovely "Summertime" song. George played with the most beatific smile on his face. He seemed to float on the waves of his own music with the Southern sun shining on him. Ira sang – he threw his head back with abandon, his eyes closed, and sang like a nightingale. In the middle of the song George couldn't bear it any longer and took over the singing from him. So it went on. George was the orchestra and played the parts. Ira sang the other half. It was touching to see how he, while singing, would become so overwhelmed with admiration for his brother, that he would look from him to me with half-open eyes and pantomime with a soft gesture of the hand, as if saying: "He did it. Isn't it wonderful? Isn't he wonderful?"
Cited in: Greenberg, Rodney (2008) George Gershwin. New York: Phaidon Press, p.181-2.