Comment by ZooCow

A plausible alternative explanation for asking Johansson:

  (1) They cast the current actor to test the technology and have a fallback.  The actor sounds somewhat different from Johansson but the delivery of the lines is similar.  

  (2) They then ask Johansson because they want to be the company that brought “Her” to life.  She declines.  

  (3) They try again shortly before the event because they really want it to happen.

  (4) They proceed with the original voice, and the “her” tweet happens because they want to be the ones that made it real. 

Asking shortly before the release is the weakest link here. It’s possible they already had a version trained or fine tuned on her voice that they could swap in at the last minute. That could explain some of the caginess. Not saying it’s what happened or is even likely, but it feels like a reasonable possibility.