I find the whole concept of the Origins characters a bit of mess. For example, when playing as one of the Origins characters will they have dialogue responses limited to their general personality/alignment or will you be able to craft Lae'zel into a 'good' character through your actions/achievements? If so, that would seem to go against her general demeanour? That is not to say that evil characters cannot redeem themselves but if this path is open to every evil character then it just becomes a bit OTT in my opinion.
I think dialogue responses will be somewhat limited to the Origin character's general personality/alignment. We can see a little of this for certain character types. For example, when talking to the Tiefling couple in the Grove, its usually a conversation about what creature comforts one imagines one will indulge in when they get to Baldur's Gate. As a Gith, none of those options were available to me other that remarking how foreign those things are - even not knowing what a cat was. I assume playing as Lae'zel, when Origin stories are complete, it will follow something similar.
Whether or not you can play the Origin characters as the polar opposite of their alignment - I think the writers still want to write believable characters, so we have to just wait and see how far these characters are able to stray. All the characters are written with aspects of occupying a moral gray area, and the most evil ones have a root cause that can be shaken.
