In the original Sherlock, he and Mary get on very well. Where she’s turned up in movie versions, it is always a spiky relationship. But why would they not get on? We really liked the idea that Sherlock immediately takes to Mary and she takes to him. There was lots of stuff we never used, like she would go around for violin lessons and John would not know. They’re just great friends.
Steven Moffat at Sherlocked ‘How do you describe the relationship between Sherlock and Mary?‘ – (x)