I originally was a guitarist. But back in the day when I was in my first band, the bassist could not really play.
I remember one song we were doing.. Funeral Pyre by The Jam, the poor sod couldn't grasp it.
I recall showing him how to play the bass part and was sort of cajoled into 'Why don't you take over on bass?'
So I did.
I would probably have done the same if I had my time again.