Something nobody else has mentioned, so I'll throw it in here...
@Men in Black how is your fitness? Depending on the game, you can burn a lot more calories on the line than in the middle. In the middle, you can control your position, you can "pace" yourself so to speak. On the line, you have no such choice - you have to be in line with that second last defensive player. One of the teams enjoy playing fast long balls... be ready for lots of sprinting - don't forget you're responsible for whether the ball is over the line or not, so you have to keep pace with ball and/or players the full length of your half - far deeper than you're likely to have done in the middle. Goal-mouth scramble - that trip to the final for the winning team could well come down to your judgement call - be ready to deal with it, and make sure you're in the right position to make the call.
Practice crabbing before the game - your first proper line, and your muscles will not be used to it.
Remember, keep the flag in the hand closes to the ref - typically your left hand, but if you turn and run to the halfway line, move the flag to your right hand. Never cross hands with the flag up, always when the flag is down. Be prepared for some "bants" with spectators - you're a lot closer to them on the line, and there will more than likely be far more there at a semi-final than you will have had so far in the middle - so be prepared for some abuse.
A strong challenge goes in near you, yet the ball goes forward to an attacking player. Foul or no foul? Onside or offside? ****, where was the attacking player at the point the ball got played forward, you were busy concentrating on whether a foul had been committed or not. A matter of seconds later, a cry for a goal goes up - did all of the ball go over all of the line or not? Can't tell, the confusion of the foul/no foul, offside/not offside means you're five yards away from where you should be. The ref is desperately trying to get eye contact with you, get your non-verbal opinion, but you're everywhere, losing focus. Because of the reaction of the goalkeeper, the ref awards the goal: defending team, coaches, manager, spectators are screaming at you that you missed a simple offside, the ball didn't go over the line, a mass confrontation is developing in the goal area, what should I be doing, where did the ref brief me to go if this happened, have I remembered my notebook and pen in my pocket... Maybe five seconds from foul/no foul to goal decision. Are you ready for that?
All this to say that being a NAR isn't as simple as you might think, and is in no way comparable to a CAR. NAR appointments on a semi final and indeed a final can be tough, challenging appointments, where you have to bring to play a whole other range of skillsets to being in the middle. It is never "just" running the line. Go and appreciate the semi-final appointment, often rewards for "good service" during the season, stand proud to the challenge, and come back after the game to let us all know how you got on.