So in my Strontium Dog game this week the PCs were arrested. They had been framed by a system government who wanted to invoke martial Law and had generated a Terrorist threat as a way to justify it.
The PCs had prevents the main thrust of the terrorist threat, a zombie virus on a star liner, they had brought some of the ringleaders of the rebels to justice, luckily not pausing to speak to any of them. They had fled the system when attacked by government agents.
This was a loose end so when they came back from Hell (see previous post)they were arrested.
Now I expected a daring escape attempt travel to the system track down the real story and clear their names.
they of course decided to go to trial.....
So I had to create a trial sub-system on the fly . Interspercing roleplay parts with rolls, I didn't want to have to role play their lawyer cross examining legal experts also played by me as they watched ..... for a 4 day hearing.
They had a bout an hour to prepare their case. They hired some investigators to follow up leads and they found some holes in the case agaisnt them basically stuff the government didn't know they had done that they had witnesses for....

What followed was a suprisingly sucessful session. Each side had a number of witnesses presenting part of the evidence. For each witness if the party had found evidence to counter the case i gave the defense a bonus +1 or 2. Their court appointed lawyer was crap, well they are mutie scum, the persecutor was good. For each witness we rolled a law skill for the barrister with mods and marked each one win or loose for the defence. Then when all 11 witnesses, including the PC with the best fast talk , where we role played and they got their fast talk as a legal mod, we totalled all the wins and loses in we ended up with a score, the players burnt a few Dog Chips (hero points ) to sway a few of the rolls and ended up +1 over all. Each of he 3 judges then rolled with a -4 modifier because they are all mutant scum after all (in retrospect I should have made this disadvantage rather than a straight -4).
Any roll above 13 (the standard skill check) meant an innocent vote from that judge. Burning their last Dog Chip (they burn 4 others throughout the trial) the party just won 2-1.

Very suprising ..... and not what I expected at all
The guys loved it they said it played out like a legal court case movie and the final decision came down to one last critical (re-rolled because of the Dog Chip) roll which gave it real tension. I deliberately didn't want this to be either me handwaving it or just making a single roll and this just worked.
I will make this my 'leagal case subsystem' from now on

Anyway have any of you ever ad libbed an entire sub-system on the fly how did it play out?

Pretty much how I DM... aheh...

But running the old Illithid series near the mid point the PCs picked up a sort of eyeless Gnoll with a old artifact that had a chance on critical to grab the targets brain.

The PCs ended up liking this fellow and asked to pick him up as a NPC since they were short on players at the time.
He was ment to just be a "point characters to right location" encounter. But they did need the help and miraculously he survived a deadly encounter in that area and so I allowed him to tag along thereafter.
So for the rest of the campaign I was playing this Gnoll-thing running around and about every other battle there would be a *SHLORP!* as someone got feebleminded the hard way.

Great fun.