In contrast to the earlier party where the verse (Esther 5:5) did not mention Esther in relation to drinking, this verse does. The Vilna Gaon, Alshich, and Yosef Lekach agree that the reason for this is because Esther was fasting then, but is not at this point.
The M’nos HaLevi explains that Esther did not really drink in order to keep her head clear.
A compromise opinion seems to be that of the Avos HaRosh, who writes that Esther merely pretended to drink by putting the cup to her lips.
R’ Yehonosan Eibshutz writes that Esther was more confident at this party than before because Haman had an idol on his person (see #190 above) then, and one should not pray in the presence of such things (see Midrash, Shemos Rabba 12:5), or even pictures of them (Shulchan Aruch Orach Chaim 90:23). However, Haman removed his idol before “demeaning” himself in the act of leading Mordechai on a horse throughout the town so that he would not feel degraded by a Jew. Therefore, Esther could now beseech H-Shem more confidently.
I would have thought to say that perhaps the idol was more appropriately covered by Haman’s daughter’s mess. But then Esther would still not have been be able to pray in the presence of such things (Shulchan Aruch Orach Chaim 79:1).