Personally I don't think true spontaneity is anything great at all. You can try to walk into a scene like Kareena Kapoor (famous for being "spontaneous"), and sometimes it will work, but more often than not it really is not impressive. In fact, look at Parineeti as another example; she seems to thrive on spontaneity, but only four movies in and people are already tired of her acting and think it looks the same in every film despite great differences in the writing of the characters.
I think whatever Sushant has been doing has worked really well. Each of his characters so far have been completely distinct from each other as well as from himself. Never feels like he is being fake or forcing anything, which I think is the whole point when it comes to acting!