I agree pretty much with 411A. An initial command check-to-line needs to involve a certain amount of questions, and certainly requires a demo of a couple of different types of instrument approach, as it is a hoop that one should be able to jump thru. Bit like the ATPL exams. However, once established, the ongoing line checks are, or at least should be, much as 411A says - a check that company procedures and good commercial practices are being followed.
I saw a document prepared in Australia by a CASA inspector which ran to about 30 pages on what a line check should be. It was a grilling to end all grillings. Trouble with this approach is you need to sign on three hours before takeoff to answer all the questions, and by the time you get to the aeroplane you would have a very uptight crew on your hands. Continuing the grilling in the air would be downright dangerous.
If you feel that a check captain in the jump seat is distracting you with too many questions or other demands, tell him/her to butt out until AFTER the day's flying. At least that way the flight management hopefully won't be too compromised. To a certain extent you can pre-empt too much b.s. before departure by signing on at your normal time; that way he would incur a delay if he distracted you with other demands.
That does not mean that you should not prepare yourself by reviewing limitations and procedures etc. A good checkie can find a fake anyway, so if you are substandard watch out!