In the box-seat
In a superior or advantageous position.
This phrase doesn't relate to boxes at the theatre, or even to seats made of boxwood. The box-seat was literally a box which was used as a seat for the driver on coaches. This was set at quite a height in order for the coachman to be able to see beyond the one or two pairs of horses that drew the coach. It isn't difficult to imagine how 'in the box-seat' took on its figurative 'advantageous' meaning. This English phrase has the same meaning as the American 'in the catbird seat', but although the US version was coined later there's no reason to suspect that it was derived from the earlier phrase.
The first reference I can find to box-seats is in an advertisement from The Times, June 1804:
"To be Sold, A LANDAU, little the worse for wear, with a barouche box seat."
'Barouche' is the name of a carriage with such a seat - "A four-wheeled carriage having a seat in front for the driver, and seats inside for two couples to sit facing each other". (see also Vis-à-vis)
The use of 'in the box-seat' to describe someone in a favourable situation began in the mid 19th century.