This is right Sarge. Ref's discretion.
It's a tough one to get right. If assistants get too involved its surprising how much that can affect the game and confuse players and fans. It's also not possible for any human being to reliably follow the defensive line and touchline
and catch fouls. At the other extreme, especially in the pro game where everything happens at such pace, completely passive assistants means all but the fittest and most dynamic refs will have an impossible job.
The best refs know their style and strengths and weaknesses and brief their assistants accordingly. Ego driven refs tend to tell their linos to do offside and throwins only. Wimpy refs ask for everything so they can deflect responsibility from themselves. Good refs will explain how they tend to position themselves, make it clear it's not the assistants responsibility but ask for a flag if the lino happens to catch something he or she may have missed. Yesterday's seemed pretty good.
On the subject of linos I love this quote from the Cheltenham manager yesterday after a Pompey fan had to step in from the crowd to run the line in the second half. "I think they should all have a curry and four pints before coming on, it seemed to do the trick.He was probably the best official we've had this year."