Hello /tg/, I apologize if I'm breaking protocol here as I'm not very familiar with the board.

I'm planning to do a RP session for halloween, supposedly a one-shot thing.
I'm only an occasional GM that we rely on in between campaigns to break the settling monotony of our more classic main GM.

I'd like some advices from more experimented GMs/players among you.
First of all, I'm kinda worried about the whole one-session thing.
I know our group and I know we take ages to get started.
Do you guys have any advices or comments about one-shots?
If it's doable, I'd like tips on getting started quickly and making sure the whole scenario can fit in one go.

Also, if you have any nice ideas for a spooky session, I'm all ears.

Lastly, I'm known to be on the trolling side as a game master.
My stories usually revolve around some twist, like the vilain being one of the players or everybody thinking they are the traitor in the group when none of them are actually loyal.
This kind of stuff.
This time around I was thinking about making the players think they are (really strong) humans in an alien world when they're really small animals (rats?) from the wild encountering human civilization (sewer, roads etc.).
I'd like your opinion about that.
Is it too dumb? Someone must have thought about doing that before.
Regardless, if you can think of funny situations that would maintain the illusion, I'm also all ears.

Thanks for reading that wall of text