Just like the referee, your job is to facilitate the best game experience for the players.
Have a good working knowledge of the rules so you can apply them if no referee is around.
Put safety first.
Have an objective for your character.
What do they want? What drives them? Make it tangible and obtainable. (Eg 'To steal the blueprints and get back to Moscow' is fine - 'To see what happens' isn't very dynamic.) Talk to the GO to clarify your objective(s).
In any NPC role you must know:
What you should say.
What you can only reveal if triggered.
What trigger(s) reveal that information.
What you must not, under any circumstances, reveal. (ie what gives the plot away and ruins the game.)
The strengths and weaknesses of your role. (For example, they may be afraid of falling down the social ladder and will do anything to avoid disgrace.)
Do some research or ask a colleague for specific historical details that your character would know.
Make sure your prop and costume requirements have been discussed with the GO. Be clear about what you are expected to bring.
Pay attention to personal props. Players will often search jacket pockets, coats, bags and bins! Don’t leave a copy of the scenario where they can find it.
Know where the Out Of Bounds area is.
Know in advance where all your props will be kept or set.
Let the players play the game. Don’t give them clues to begin with. Let the ref tell you how much to lead or mislead them.
If dead, stay out of the way – preferably in the Out of Bounds area. (Please don’t play football with other dead NPCs in the garden. The players are still in the experience.)
Bring a book and a torch. NPCing involves waiting. Make sure you are warm/have food.