When we saw him drinking the flask, he’s not his usual flamboyant, theatrical self. I think, at this point in time, he’s tired of being the Dark One, and I think it’s safe to say when this episode happened, it was long before ‘Skin Deep.’ He’s probably still tortured by the way he treated Belle but he still has to put on a show as the Dark One.
Thus, I think whatever is in the flask, helps put him in the right frame of mind to perform as Rumpelstiltskin.