The way they did this scene in the show was really silly. Who drinks poison in front of the person they are trying to poison so that said person can see as they are dying of the poison that the cup is poisoned? That's just asking for fail.
The book version has her telling Cressen before they drink that he doesn't need to do this which is in my opinion a better way to do it. It hints that she already knows there is poison in the cup without making Cressen look like a fool. It also sets up her up as a grey character as opposed to a completely malevolent force as the show seems to want to present her as. She seems to feel sorry for Cressen. I really wished they carried that over to the show.
Yeah definitely. The line in the book is that she watches him with pity. we didn't get much of a sense of that on the show. But i think they have plenty more chances to shape our perception of Melisandre in future episodes and seasons.