Freddie being all cute and hard at work, 1975 :)
Moffat: we need someone to play John's wife.
Moffat: wait...Martin has a partner.
Moffat: but who can we get ot play Sherlock's parents?
Moffat: wait...Benedict has parents.
Moffat: where are we going to find a child to play young Sherlock?
Moffat: wait...I have a child.
Moffat: man I'm good at this.