For those unfamiliar with what the 5 Act Structure is here is a handy link. It's the structure most commonly used for writing hour long dramas, Shakespearean plays, and many other longer forms of art that still want the structure and beats of organised story telling, but with a little more depth.
I'm currently running a hybrid of API (Apocalypse Prevention Inc) & Agents of Oblivion. Both of these are more or less games about government"ish" agents that investigate and stop supernatural plots that could end the world. I feel like I should share this as it informs the style of adventures I'm writing, but with a little tweaking most or types of stories can fit into this formula. Here is the break down for what I've started trying so far...
Act 1: Introductions
The heroes are given their mission briefing, choose their gear, and we arrive in the starting location (Usually a foreign country, but new city or small town work as well). This act is the quiet before the storm so let the Location be the star with colorful NPCs and some fun roleplaying. End this act with the first appearance of the threat either by the heroes digging them out or some kind of ambush confrontation.
Act 2: Rising Threat
This act opens with the resolution of the conflict that ended Act 1. Followed is the hunt and build up of the adventure with at least 3 small scenes each either escalating the danger or revealing more of the villain's plans regarding the threat. At least one decent threat should encounter the characters, but not as deadly as the climax.
Act 3: Climax
This is the turning point of the adventure where life is either going to get much easier or harder for everyone involved. The characters should finally meet the true threat of the adventure leading to a challenging conflict. The Villain behind the threat may get away (a)or be defeated here(b) and the greater threat is either thwarted(A) or the heroes fail to stop it(B).
Now the next acts are incredibly dependent on the outcome of Act 3 so I've broken them down based on that outcome. The one big note for all of these is to save that final fight for Act 5.
Act 4: (a,A) The Hunt
In this outcome the Threat was thwarted, but the villain got away. This act should be about hunting down the villain as he scrambles to regather was power he may have left. Do your best to keep the villain just out of reach for a couple scenes as the heroes clear out the remnants of a now weakened, but still deadly, force or follow clues as they slowly catch up to a now obviously weakened villain.
Act 4: (a,B) Settling the Score
In this outcome both the villain got away and his plans succeeded. This is possibly the worst case scenario as the villain's goals succeeded and technically lost. Now it's a race against time to find some way to get one last shot at the villain. This should include finding some way to even stand against a much more powerful force. This should be an uphill battle for the heroes as they struggle to find some way to set things right.
Act 4: (b,A) Clean Up
In this outcome both the villain was defeated and the threat thwarted. This is by far the easiest for the heroes as there is no single big bad anymore and the threat is gone. The trick is a truly good villain has back up plans and some of those plans are still out there. This might mean cleaning up remaining splinter cells of goons or the hunt to find and stop some horrible back up plan that the villain reveals in their dying breath.
Act 4: (b,B) Righting Wrongs
In the outcome the heroes defeated the villain, but not before his plan was completed. This is all about finding ways to stop the outcome of those plans before all is lost. This is similar to Settling the Score, but in this case the outcome is loose on its own. This might mean oncoming war, a rampaging beast, or some other disaster that doesn't require the villain to actually be around to still be an issue. This is probably the rarest outcome, but something worth keeping in your back pocket for a nice shock when the heroes think everything is wrapped up.
Act 5: Final Round
This is the last big scene of the adventure. I can't go into too much on specifics as Act 4 will heavily influence what this actual scene is, but there's a simple rule of thumb. If the threat was thwarted this should be a simpler encounter or scene than the climax and just about tying up loose ends. It should still be a challenge, just not as much as what they had already accomplished. This is more or less one final victory lap for your heroes so make it good. If the heroes failed to stop the threat this is the big final fight. This is when everything is against them and even with whatever they may have gathered during Act 4 this will still be difficult. Basically this is here not only to end the adventure with a bang, but for the heroes to feel the weight of their failure in Act 3.
After that it's simply wrapping everything up. Obviously if the heroes fail in the last act things will not be good, but these are heroes and we expect them to win, even if it's at some serious cost.