Behaviours and interactions are only achievable for third-party developers on watchOS if someone at Apple has already invented them and exposed a checkbox in Interface Builder. Any dynamic transition or animation in a WatchKit app is basically impossible.
For example, you can transition a table row to a new appearance if that row changes height because WatchKit happens to support that. But if you want to cross-fade the contents of a row that has the same height before and after, you are out of luck… More here.
Can’t help but think that only very simple, very popular services can get success from watch apps. Then again, until the time that we are not carrying phones with us all of the time it may not happen for anyone.