when a marine places a guard order does he choose an invader and then he makes that invader walk into his line of sight?

No, how Guard works is that if the INVADER PLAYER moves an invader to the marine's line of sight, the marine gets to do an interrupt attack.

And it is a good idea to make that attack when one monster enters the square occupied by another monster - one shot two kills

Has it been confirmed that a guard order interupt attack can only be used when a monster that was 'out of sight' moves into the line of sight? The rules have this as an example but the written part does not make any mention of this 'moving into line of sight' requirement.

I've played through the first 2 campaigns as the invader with my friend as the sole marine. He has the marine card that allows him to place a guard order if he advances and it's like he gets an extra attack every go; although without this I think he may be struggling to survive alone!

Wait, you have to wait for an Invader to move into your line of sight before activating a Guard attack?!? As in Space Hulk's Overwatch? When did this happen? I've always played that the Marine can *whenever* he wants discard the Guard order and just attack, regardless of what the Invader does. Heck, he can do it before the Invader draws cards/spawns even. Interrupting a turn is interrupting a turn.

(from page 10 in the rulebook)
GUARD
A marine that has placed a guard order may make
an interrupt attack .

At any point during the invader player’s turn, a
marine may discard his guard order to immediately
interrupt the invader player’s turn and make one attack (following
all the normal rules for line of sight and attacking). The
invader player’s turn is immediately halted (even if the invader
player was about to attack with an invader figure), allowing the
marine to resolve his interrupt attack. After the interrupt attack is
completed and any casualties are removed, the invader player may
continue his turn.

The invader player must allow for an interrupt attack at any time,
and must reverse any movement/attack if it was made too fast for
the marine player to have a chance to declare an interrupt attack. If
a marine player declines to make an interrupt attack, however, he
may not change his mind later.

A guard order stays with a marine until discarded by one of the following
events: 1) the marine takes 1 or more wounds, 2) the beginning
of the marine’s next turn, or 3) the marine discards the order to
make an interrupt attack.

There's nothing in the above text about waiting for an Invader player to move a figure, or even a figure being in LOS. If that were the case, the marines could never lob a grenade at an invader around a corner. And the "example" doesn't say anything about having to wait, it just says that the marine decided to wait so he could shoot him. AFAICT, there is no such rule that says Guard has to wait for monsters to move within LOS.

-shnar

Schnar we've played it like that as well, and that's how it's meant as far as I know. You can even waste your interrupt when there is no invader on the board if you would like to do that. It'll just be pointless, but allowed.

Pretty much the same as shnar, only thing we changed was that Panic trumps Guard (when we still used base-game Invader deck).

Cool, seems we've been playing it right then. I just got a little thrown when I came to this thread and read what others had put! Who ever said the internet was a valuable resource...?

We too have been trumping the guard attack with panic.