Cesare and Micheletto Preassassination
http://www.youtube.com/watch?v=iI-eowZVzGM
[Front-paged - stabguy]
This is a continuation of a video where I prevent a scene from happening in Calling All Stand-ins memory. In this video I preassassinate Cesare and Micheletto.
Cesare: His AI is programmed to back away from you (because of the mercenaries fighting and you probably have to get in open conflict). Use this to lead him to where there is water. When you get there you can't make him back into the water so you have to grab someone and move that person at Cesare to push him into the water. You can probably also use a horse to push Cesare.
First here is how to prevent the scene from happening.
http://www.youtube.com/watch?feature=player_detailpage&v=B5VDQ12oa0A#t=340s
Basically go to the building by the haystack and hide behind it (or else the scene will start). Then lock onto one of the papal guards and send a group of mercenaries after them. Immediately run away and if done right it will say "target located" without the scene starting.
Micheletto: His AI can be manipulated. First move him out of the area where you start the scene. You can't do this if he's fighting mercenaries. This might not seem possible because you have to send mercenaries to prevent the scene, but there is a workaround. If you run far enough away when you send the mercenaries they will disappear when the target is located. Micheletto will shove you when you get close to him. He walks toward you to shove so use this as a way to move him out of the area. When your out of that area his AI is just like a guard's. You can get him to fight you by stealing or killing civilians. I'm unable to do any kill animations on him, and some other animations get weird (countering with fists, disarming, kicking). Lead Micheletto to water and make him fall to his death by blocking his way when he jumps to platforms. Even after he falls in the water it still shows his SSI arrow for a little while.
EDIT: I just recently found out his AI can be manipulated without moving him out of the area. Here's what to do after the scene is prevented: Go to the area (make sure there aren't any mercenaries fighting him), go find some guards to fight (not too close to the area), come back to the area and Micheletto will attack you on sight.
I also included a few more things. You can make Micheletto flee by locking on and attacking. Since he doesn't hold a weapon he flees pretty quickly. Also if you become anonymous afterward you can still taunt him if you're locked on (same for the papal guards).
Castello Preassassinations: In my original video for preassassinating the targets at the Castello I didn't get all of them. I tried again and put it in this video. 
What you need to know is you normally you couldn't get the targets because attacks go through them and they disappear. However if you tackle one of them you can do a prone assassination (in the original video I did an air tackle, but in this case I did a regular tackle which is better since it allows you to get in the huddle and it's faster). Assassinating one of them messes up the AI a little bit in that they don't disappear unless you get too close and attacks don't go right through them. Getting the other three in one hit is a challenge because after the two walk to Cesare he walks through the gate. Also attacks seem to go through them while they are walking, so the time to get all of them would be right when they stop, right before Cesare is on the other side of the gate.
so how do you start this exploit out? what i mean it, how do you get close enough to kill cesare and michelleto without starting the scene?
Guess what? CHICKEN BUTT 
Oops, I forgot to show the setup in this video. 
I originally showed it in another video. Just put the link in the video description.
Just to clarify here are all the times Micheletto appears in the game and whether or not he can be killed.
1. During the villa attack, in the scene where Cesare kills Ezio's uncle. Micheletto is in the background, along with the Baron and Banker. The scene starts after you fight groups of enemies. While fighting them you are trapped in a small area with no way of getting out. There doesn't appear to be any way of getting to Micheletto in this case.
2. When you infiltrate the Castello the first time. The last four targets appear in a scene. You can get him with a tackle and prone assassination. It's also cool that you can get in the huddle and kill the other three targets with the throwing knife special attack.
3. In the beginning of Calling All Stand-ins memory. You can prevent a scene, then manipulate Micheletto's AI so that he fights you. Lead him to the water nearby and block his way when he jumps from platform to platform. He falls in the water but doesn't drown immediately (it still shows his SSI arrow). He does disappear in a short while though. Since the scene didn't start you will have to restart the memory to get farther in the game.
4. After that scene you follow Micheletto. Killing him desynchronizes you, but there are opportunities to get him. There are a few scenes with him talking to his allies who you have to kill afterwards. There's also a scene when he enters the Colosseum. These all are opportunities to get him. In one of my videos I kill him by throwing a weapon. What I do is get close enough to lock on to him without starting the scene then throw the weapon. Even though he dies the mission doesn't fail. However I'm unable to move. I've tried getting a little closer to throw the weapon, but the scene starts before it hits him. What you need to know is the scene starts when you are close enough to lock on to his ally, the character you have to kill after the scene. Now I've been able to get close enough to lock on by hanging from a structure to hide from view (the scene will start immediately if you lock on). I've used this to call the assassin recruits, but that hasn't helped much. The mission will fail if he dies. I tried starting the scene after calling for an arrow storm, but that froze the xbox360. I also tried running away after calling the assassins, but that didn't prevent the scene. Here is a challenge: Get a heavy weapon and a long weapon in him. If you race ahead of him to where the scenes start the scene will start at a certain time (probably based on where you and Micheletto are) so it could be possible if you know when to throw the weapon. But the easiest time to get a weapon in him would be when he enters the Colosseum since it happens at a certain time and theres even a nearby group to blend in with. Another cool thing to do is the smoke bomb weapon grab detection exploit. It's just funny if you drag him with a weapon at his throat, then a scene starts and Micheletto acts like nothing happened. So there are multiple opportunities which might count as multiple preassassinations.
5. He can be killed early on stage in the Colosseum. Just wait till he's done talking since he's animated and invincible to attacks during that time. You'll have to use a ranged weapon though.
6. During the assassination if you throw a weapon at him at the last possible moment he will survive during the scene with the weapon in him. You can also kill him after the scene while it desynchronizes and it will still show the assassination memory corridor scene (yeah, you still fail the mission). It would be cool to do both in the same assassination: Throw the weapon at him from not to far away, then when the scene ends assassinate him again (either kill him with the hidden blade or get the weapon out of him and kill him with it).
Then of course he survives the assassination:
Micheletto: "I am not yet dead."
Ezio: "I did not come here to kill you."
He supposedly dies later (perhaps in a future dlc?), but we know better.
Nicely done! I do, however, have my doubts about the pre-emptive death of Micheletto really being a death.
What seems to happen with Micheletto is that he falls through the water, then gets stuck somewhere under map, where the line of sight is broken. This is indicated by the SSI still being visible and your status changing to 'broken line of sight'. After a while, Micheletto (displaying guard behaviour) gives up.
This is further strengthened by the fact that we know that the map is 'vulnerable' there, as Stabguy demonstrated in one of his Romulus Lair videos.
To conclude, it's pretty likely that Micheletto was still alive even after you threw him in the water and he 'drowned'. This raises a question: if Micheletto didn't die, then did Cesare? Cesare seems to act as a modified civilian. The only way to get those to fight you or something is to steal from then, maybe that's an option?
You made a good point. I did fight Micheletto again and drowned him, this time while in open conflict with other guards. The red arrow went away after a short while. He doesn't drown right away because he is invincible from things that would normally kill other characters.
In Stabguy's The nine lives of Dante Moro video the same thing happened to Dante in a preassassination.
http://www.youtube.com/watch?v=0fnELeIz9i8&feature=player_detailpage#t=221s