Funny to see you hear Chrystal! It was your game that inspired me to start a group game, I have decided this summer is the time to do it.
So if I ran a freeform game as I think is most likely, what is my check list so to speak of things I need to have ready.
I already presume, setting, plot, my character, other NPCs...
Surprising as it might be from a guy who has "freeform, unless for one-shots" in his "OFFs", I've actually played and ran freeform games. The change in GMing approach is zero, you just have to compensate for the minimalist, easily abusable system that people call freeform.
OTOH, "sandbox" is how I prefer to run, so maybe I could tell you a couple things.
First, let me join the chorus:
GMs are well-advised to not have a character they could call "theirs". However, sometimes that's not viable, as the GM wants to play as well, or there are few PCs - or the PCs simply hire some NPCs and take care of them! I've done it as well, without major problems.
My answer is to not have a full
character. Have your guy be a more minor player character. If they're all veteran SEALs dealing with the supernatural, make him a soldier straight out of basic camp. If they're a group of veteran dungeon-delvers, have him be the guy that bears their torch and fights with a mace only if approached. Either of these might be talented, and will have time to interact with the others, and is trying to be helpful, but not succeeding often. In a way, he lets them be the stars of the show. Don't worry, you get to play everyone else.
Don't forget that as a GM, you know all the relevant info and all the solutions. The key to many problems would be to find the info and devise the solution, and that goes doubly with freeform. Need I explain how you might come off as coddling your character?
Also, make him the ally of the player characters. Best way never to test whether he can be killed without your agreement is to never have a PC wanting to kill him. Make him helpful and not a spotlight-stealer. I've had players voting for "my" character at the end of the session when we decide who gets an extra improvement, despite him not even having PC status. Guess they liked him
I didn't actually win the vote, but it wasn't too bad for someone who wasn't supposed to be a participant.
Second, that was enough on your character. If you want to run sandbox, put setting and NPCs before the plot! The players will change it anyway, if they're proactive, and they should be proactive for a sandbox to work! (A sandbox where nobody plays in the sand because everybody is waiting for someone to shift the sand for them is a sad picture indeed).
Actually, my absolutely best advice for a sandbox game. Put "looking for players that want to be proactive" in your game advertisement! That includes having goals and chasing said goals.
After you have the player characters, just present them to the setting, and give them NPCs to interact with. If they're proactive indeed, they'll start supplying you with new plot, simply by having goals and chasing them. In the effort, they should have to build alliances, create enemies - bingo, new motivations for your NPCs!
And of course, you might still have some events that are just going to happen. Although that also depends. An invasion from another dimension is one thing, but an invasion from the neighbouring country would logically have at least some signs. Let the players notice them, if nothing else. They might even warn the authorities, and make them listen. Or they might prepare a resistance movement in advance. Or they might sell off to the invaders.
After all, you may pre-determine the events, but the reaction to them is up to the players
. Just go with the flow, and you'll find it doesn't mean losing control. It means remaining in control without struggling for dominance. Well, this might change for some naughty scenes, but I'm sure you get the idea
Anyway, it's almost time to run my weekly session. Hope that was helpful!