I'm curious about what about her acting gives you that impression,
Her acting in general obviously.
Her line delivery, her emotional expressions, her interpretations of roles, and her interactions with other actors in the films she performs in.
since I actually like that they cast her because of her acting capabilities for this role:
Well, what you like is not a metric for which to determine why they made the casting decision they did though - and unless you have some info I don't (which is perfectly possible mind you), there is nothing to say that they picked her specifically for her acting capabilities, and not for her ability to draw a crowd as a blockbuster actor.
(her physical capabilities from the MCU stuff as Black Widow, and her ability to present techno-speak from Her being the most outstanding things that come to mind immediately).
I'm simply not impressed by that, and it doesn't strike me as very relevant.
Whatever physical feats she did, if any of the noteworthy were even her at all, and not a stunt-woman, are not outside the domain of any average able-bodied woman given the training all actors inevitably receive before playing a movie like that.
This is not Jackie Chan level stuntman-ship.
As for her presenting techno-speak - what was so outstanding about it?
What about her techno-speak made you think that was a particular poignant recital of lines above and beyond any other average recital of techno-speak?
My biggest issue with Scarlett Johanssen is that I simply don't think she has the ability to act with the subtlety this particular role demands.
In this instance, which I think is apparent even by the teasers, she's far too expressive and animated for this particular role.
For the most part, Motoko has a face like a stone-wall, which is only brought to life at times through faint hints of expression.
Scarlett moving through the hallway with her gun raised looks stressed and unnerved - much too human - which is what I would expect from Scarlett because she simply isn't, IMO, an actor whose caliber is high enough to absorb the character.
I could be wrong though - she could be able, and simply denied a chance to do so due to bad directing.
That being said - these teasers did little to change my mind.
To me, she looks completely out of place, and she does not, in me, invoke the feeling of being an android.