There isn't too much difference between the inside and outside centre.
But as a rule the outside centre tends to be quicker, but not maybe as big as an inside centre.
England's Mike Tindall, who has played most of his international rugby at outside centre, has the ideal build for the number 13 shirt.
He's a hard, straight-running centre who tackles well, who takes the ball into contact well, which perfectly complements the skills for a number 12.
It used to be that the centres had an understanding but these days the whole backline has to have an understanding.
As a coach, you run through repetitive exercises, drills and playing alongside one another to work out what you are going to do.
If you don't then there's no rapport built.
Hopefully you get enough opportunities to work out what each other are going to do.