The conceit of a spin-off is that the new show and its predecessor exist within the same reality. For Private Practice, this means that the woman who, in our reality, is Kate Walsh is actually Addison Montgomery within the reality of Grey’s Anatomy and all its descendants. She is Addison Montgomery whether she is in Seattle, LA, Alberta or Auckland. Were Kate Walsh to play a different character within the world of Grey’s Anatomy (which includes Private Practice), it would mean that Addison must have a long-lost twin walking around somewhere.
This rule, if broken, has the power to undermine the credibility of the world that the writers have created.
So imagine my surprise (or lack thereof, considering how much faith I have in Shonda and co.) when I finally get around to watching the season finale of Private Practice and find that Tessa Thompson is guest staring- as someone completely different from her role as the Chief’s niece Camille on Grey’s Anatomy. Oh well, Addison’s always so caught up with her own drama maybe she didn’t notice that Naomi’s patient Zoe was identical to an important person in her life back in Seattle.