An acting scrum-half is a Rugby player who temporarily acts as the scrum-half, due to the regular scrum-half's unavailability, due to injury or sin-binning.
Vicente Sánchez
Fourie du Preez
practice, practice, practice
There is, however it is sometimes named halfback.
The three quarters are the wings and centres positions. This is connected then by the half backs who are the scrumhalf and flyhalf
No. 9 is the scrumhalf and this is the "quarterback" of rugby. He/She is the go between the forwards and the backs.
Whilst the term "spots the ball" does not appear in the phraseology of rugby I'm assuming that you mean a Scrumhalf
Halfback or scrumhalf is usually the smallest player on the field. Depending on certain skills, small players can be located anywhere in the back line
acting, is acting, I am acting, he/she is acting, they are acting
good acting and bad acting
acting head
Impromptu acting is acting at the spur of the moment.