In pelilauta there was a topic about "GM improvisation" and I posted my response there. I decided to roughly translate my thoughts and blog it also. You can find the topic from the following link (in Finnish, but Google translate can help): http://pelilauta.fi/index.php/topic,459.0.html
When I start running a game I decide the starting point and ending conditions. Ending conditions are those what determine when adventure or campaign ends. Dragon is slayed, village saved, beggars banished, taxes collected...
What is in between the beginning and the end is what I usually and mostly improvise and is determined by player's decisions. I might have some ideas for scenes or alternative smaller quests what I want to include. I fit them on the adventure/campaign roughly in a timeline or where they fit. Sometimes between the gaming sessions I might get random ideas I include in the game if they fit it.
Also sometimes ending conditions might alter during the game. Player might take a different direction instead of what I have sketched and instead of wanting to kill the evil they might see the evil as better than they original mission provider and switch side. I let it happen and then the ending conditions might change. This is an example what might happen:
In the beginning player characters get the quest to kill the bad guy thus the initiative ending condition is that the bad guy is killed. I might have few ready made scenes, side quests or "hotspots" to visit what I want to use. I include them in the game if they fit or might ditch them and improvise new plots what will suit better depending on players' actions.
When the ending is near and player's are killing the bad guy what would end the adventure suddenly the baddie suggests that it is the original quest giver what should be killed. Players make their decision to do so and now the ending condition is changed. Instead of killing the baddie adventure continues with possible new ending. This new plot might take place during the adventure or in the finale depending on did I find new ways for the adventure during the game (improvising the plot) or did the players' choices alter it.
My adventures and campaigns live during the course of the game and usually I don't have ready material that much. More like sketches what are possibly changing during the game. Some scenes might be more strict than others but I always have space to move my campaign to different directions. Also players' actions might inspire me to change the original plans in a different direction. Good example is when players don't catch the hook of an adventure but decide to do things differently than meant or take a different course rather doing something else. Sometimes I gently try to steer players to the right direction - avoiding straightforward railroading - but if it doesn't work I have to improvise new plans for that session and possibly for maybe whole adventure.
Improvised sandbox gaming I can do well but there is a problem I have in longer period of time. It's hard to find focus for the campaign and game might stretch too long to finally get boring. Usually when I play sandbox I start with that style and during the game start to take a direction for the game. Or between adventures I might let players to play in the sandbox doing what they want to do. (One of my Praedor campaigns was kind of a sandbox. Ended after 2500 pages of A4 in 12pts Times new roman font. But in the other hand some of my more directed campaigns have been really long).
When I write an adventure I have a problem with time consumption. If I estimate that the adventure will take 4 gaming sessions to finish it usually overlaps it to 10-12 sessions. This is the problem because improvising game is in my blood and I don't usually have patience to follow the original plans. I am re-learning the skill to run an adventure without extra rambling.
If I had a different - possibly more focused gaming group - I think I'd play adventures like adventures. To start and finish in a time. Not accidentally growing 4 sessions adventure into 100 sessions campaign. Now I am starting to try out several games I have collected and I must focus on the actual adventure instead of improvised campaign. Otherwise I will never play as many games an adventures or campaigns as I want to. That is not forcing into a gamemastering mold but rather decision to sometimes keep focused to provide interesting and thrilling story instead of a novel.
My current Vampire: the Masquerade game was supposed to be second part to last game with same character starting where another ended. But darn Vampire is so fun game to play. I got out of focus and instead of running this Nosferatu adventure part 2 I improvising tangled myself accidentally in this campaign.
No wonder my head is exploding of ideas I don't use when I get stuck on what ever I am currently playing. I have to focus.
Focus or not, roleplaying is sweet ass hobby.