Disclaimer I do not own Harry Potter or BBC's Sherlock, I'd be rich otherwise... I'm only playing in their sandbox. Also I have read hundreds of fanfics so if you see something that looks familiar it's not done on purpose.
Also this is my first fic. I apologize for any errors you may see please politely point them out to me and I will try to fix them.
On with the show! :)
Reincarnation. It takes certain parameters for someone to be reincarnated. The first being you have to have died before your time. That of coarse isn't the only thing, otherwise lots of people would be reincarnated. The second being you have to have formed a very close connection with at least one other person, and that person has to still be alive when you die. There is one final requirement to be reincarnated, you and this other person or persons have to been making a difference in the world in some way. As a side note to that final requirement, the death of the first person must disrupt the continuation of the work. If for instance the surviving member of a very close partnership at a police station can continue on working without their partner then they will not be reincarnated.
Your wondering why I said they and not he/she… that is because they will seek each other out, to repair the damage that has been done. Soon after the one dies the other or others will follow. Now this is not immediate, destiny gives you a period of time to get it together, a year at most. This gives destiny and fate time to find a place to plant you into a new body that will inevitably end with your group together again.
As far as reincarnation it self, you don't remember your old life for a time. Everyone is different in this respect, it depends on how close your base personality is from your old life to your new life. The closer they are the sooner you'll remember. Now if you had a very strong personality trait in your past life it will soon transfer over into your new life; for instance if you were an adrenaline junkie in your old life you would also be an adrenaline junkie in your new life. These are your core personality traits, they make you, you. Then you have your secondary traits which can be different in your second life. It's the whole nature versus nurture argument. It's a bit of both. Your core traits being nature and secondary traits being nurture.
Now that you have a better understanding of reincarnation let me tell you a story about a man who instead of forgiving his best friend and going back to a semblance of normalcy after his wife died continued to ignore him and grieve for his wife. Thus setting in motion a chain reaction of one of the biggest groups to be reincarnated in thousands of years. And it all started because of one John Hamish Watson...