NPCs should have a sense of self-preservation. Don't just line them up willingly, ready to be killed by the PCs.

Some games have morale rules to handle this. If an NPC is in danger, the GM makes a roll against some sort of bravery or loyalty or morale stat, and the result determines if the NPC stands his ground or turns tail and flees.

As a PC, you naturally want to encourage your enemies to hang around and fight, rather than flee. So you should do stuff to improve their morale, like complimenting them on their armour or offering them chocolates.

Transcript

Darth Vader: Report.
Captain Khurgee: Nobody's come out, sir. I don't understand.
Darth Vader: Your welcoming crew is remarkably well-armed, Captain.
Captain Khurgee: Thank you, sir.
{beat}
Captain Khurgee: Er, the welcome leis and champagne are on their way.
Darth Vader: Tear this ship apart. If you don't find anything, eject it and let the troops use it for target practice.
Captain Khurgee: Yes, sir!
{Vader leaves}
Darth Vader: {while walking away} Don't bother getting off first.
Captain Khurgee: You heard Lord Vader! Tear this ship apart!
Clone Trooper: With what, sir? Our bare hands?
Captain Khurgee: Well, I assume so.
Captain Khurgee: I'll... stay out here and supervise.