The only "official" statement that I know of governing Ghosting is the phrase seen on the "Completed" summary when we finish a chapter..."You tried to leave no trace of your presence or raise any alarms." A lot of conjecture has risen from that vague description. In particular, the incorrect notion that any interaction with the environment will give Opportunist Points. Having read so many discussions about which actions detract from a Ghost rating, I decided to conduct some research into the matter. Below, are the results of many hours of experimentation to determine the real Opportunist penalty received while trying to Ghost a chapter. All data has been verified by actual performance and checks gathered from all chapters*.

In this guide, the following abbreviations will be used:
OP = Opportunist Points.
SC = Suspicion Count

1. Light Switches:
   
   • Turning off lights with a switch will, by itself, not cause OP (Opportunist Points), as long as no one notices. However, if a hostile does see the lights go off, he will become suspicious and his Threat Meter comes on. This usually adds to your SC in your Journal's Progression Stats, as well as adding OP.
     
   • Turning lights on will normally not cause OP but again, if a hostile notices, you'll get SC and OP. In some areas, you have to be careful. Hostiles may not be around at the moment but, can appear later on. If they notice the lights have been turned on, you can get OP. Fortunately, there are very few places where you would want to turn lights on. But, if you do, turn them back off, if you can.
     
   • Using an arrow to hit light a switch is the same as pressing it with your fingers. It's just from a distance so you won't be seen as easily. However, if noticed by hostiles, it will raise suspicion and add OP.
     
   • Sometimes you'll enter an area where hostiles are already alarmed (Threat Meter is white). Be sure they are well out of the area before turning off any lights, otherwise you'll risk getting both SC and OP. If you turn a light on or off with a hostile just around the corner or in the next room, they are likely to notice the change and become suspicious, giving you SC and OP.
     
   • In one chapter, there are certain light switches that, when turned off, will actually remove OP and improve your Ghost rating.
2. Open Flames:
   
   • Wall fire baskets, ground fire baskets and campfires will almost always causes OP whether hostiles notice it or not. Re-igniting it with a fire arrow doesn't remove the OP. And, If doused in the presence of hostiles, it also causes SC.
     
   • Candles are the same as all other open flames. Putting one out causes OP even when it goes un-noticed. When hostiles notice, you get SC, as well.
3. Doors, Gates and Chests:
   
   • Opening Doors or Gates. If it's a run-of-the-mill door, it's normally okay unless you open it when a hostile is on the other side. Of course (there is one exception in chapter 6 where opening a door with hostiles on the other side does not alarm them, if timed right). However, there are certain doors that are unique and will alarm hostiles when they notice that it's been opened. Even if they don't see you actually open it, when they acquire it in their field of view, they will become alarmed. Penalties vary from SC to Detection. Generally, if there are (or will be) hostiles who could see the door has been opened, just close it behind you.
     
   • Sometimes, to open a door, you have to operate a wheel, pull a lever or clip a wire in a wirebox. In these situations,doing so will not cause OP. These are actions required to get through the door and there is no penalty for using them.
     
   • Closing Doors. Very often, opening a door causes no alarm but, closing the door will be heard by a nearby hostile. If opening the door causes no alarm or suspicion then, you should just leave it open. If you really want to close it, wait for the hostiles to get far enough away so they don't hear it close.
     
   • Cabinet Doors, if left open, can sometimes alert passing hostiles to your presence and cause SC and OP. On the other hand, if no hostiles are (or will be) around, they don't cause any problem at all.
     
   • Chests. Most of the ones I've left open for patrolling guards to see, have not raised any concern at all. Edit 10-23: Exception- Chapter 2, near the end of the Mfg Area with the locked door. Leaving the chest open (near the sleeping guard) will alarm the patrolling guard. That will give SC and OP.
     
   • Safe Doors. In some areas, if you leave a safe door open after taking its contents, a patrolling hostile will see it and become alarmed. Even if it doesn't raise suspicion, it will result in OP.
4. Locks.
   
   • When picking locks, if you miss a set point (brief red halo around the indicator) it causes enough noise to alert nearby hostiles. If someone hears it, they'll become alarmed and you'll likely be detected.
5. Wheels:
   
   • Generally, if turning a wheel is necessary to continue on in a chapter or to find loot or collectables, it's okay to turn and will not cause any penalty.
     
   • Wheels that open doors or locks are no risk and cause no penalty.
     
   • Wheels that turn off water sources that reveal loot are okay, too.
     
   • Wheels that flip bridges are part of the necessary progression of play and cause no penalty.
     
   • Wheels that turn off gas can sometimes cause OP. Gas is harmless unless lit by flame or blast arrows so, just leave them alone.
     
   • Wheels that turn off flames are usually okay, if they are required to get through an area that would otherwise kill you. However, in chapter 6, when escaping across the bridge, there is one wheel that turns off a flaming gas jet, that will cause OP
6. Levers.
   
   • Levers also have different purposes with varying results. They're typically used to open doors, turn off water sources or release walkways.
     Edit 10-18: All levers I've tested have caused no OP, with two exceptions... In chapter 5, when descending the Old Prison Stairway, the two levers that are necessary to open gates so you can proceed, indirectly cause OP. There is, however, a way to remove the OP caused by these levers.
7. Arrows:
   
   • Rope Arrows, for the most part, don't create suspicion or even raise an eyebrow with anyone. However, if it's shot to a beam that is close enough to hostiles, it will cause alarm. SC may be incremented and you will get OP.
     
   • Water Arrows and Choke Arrows, when used for putting out open flames, will almost always cause OP, even if it doesn't raise suspicion. Oddly however, when using a Choke arrow on a bird or dog, it causes no penaly at all. It only shows as a "Knockout" in your stats. But, be careful not to be too close to a bird when you shoot it. You could kill it, which counts as Predator points.
     
   • Blunt Arrows are often used to hit a switch from a distance. In addition to turning off the lights, they make noise on contact and can alert nearby hostiles, thus giving you SC and OP. They are also used to shoot the tackle blocks holding up large crates, bundles of lumber or chandelliers, thereby dropping their load to the ground. This too usually causes OP.
     
   • Fire Arrows are best used for setting Freaks alight. But, they're not much use to Ghosters.
     
   • Blast Arrows also are normally not used by Ghosters, either.
8. Trap Wireboxes.
   
   • Surprisingly, most have proven to be safe to disable and cause no OP. These are a few I've tested and verified as safe to disable... The trap at the safe in chapter 3; The trap in the underground tunnel in chapter 4; All five traps in chapter 6; Both traps in chapter 7.
9. Miscellaneous:
   
   • Cranks: A few things can be released or lowered by striking a crank or shooting it with an arrow. Unless a hostile is nearby to hear it, there will be no OP.
     
   • Tackle Blocks: Used to suspend crates, among other things, usually cause OP, as in chapter 4 or 6.
     
   • Creating distractions: By throwing a bottle in the area of a hostile, it creates a distraction and usually causes suspicion but, not always. It depends on where the bottle is thrown. However, it will still cause OP.
     
   • Moving through water will only cause SC and OP if you're seen or heard. Be sure to crouch-walk slowly. <Ctrl> + <Movement Key>.
     
   • Flash Bombs: This is one thing I haven't actually tested. Mainly, because it's going to raise suspicion, at the very least. Edit 11-13-14 Flash Bombs will cause Predator points.
     
   • Running is noisy and if hostiles, even if they're in another room, hear you, you'll get SC and OP for it.

• Things that never cause OP (unless hostiles see or hear you). All the ones I've tested are safe and cause no OP.
  
  • Picking Pockets (only one exception. See Edit remarks below)
    
  • Picking Locks and missing a set point
    
  • Other Wireboxes:
    
    • Wire boxes that allow you to open doors.
      
    • Wire boxes that provide access to elevators.
  • Switches that look like light switches but are used to call elevators or to open doors.
    
  • Other types of switches such as round wood switches in walls . Also, stone block switches that disable traps and are necessary to press in order to continue progressing through the chapter.
    
  • Wrench: Used to unscrew vent covers or remove City Heritage Plaques, the wrench, by itself, is safe. I've never had it cause OP.
    
  • Razor: Another tool that never raises suspicion or gives any penalty.
    
  • Interactive bookcases allow access to loot, collectables or the next area in a chapter. None cause OP. Yes, even the one in chapter 4 on the 2nd floor (with the guard and the maid).
    
  • Taking Damage: If you're burned by a flame, choking on fumes or even hit by a trap arrow, you can take damage and not worry about OP. Damage has never caused a penalty for me.
  
  Edit 10-15-14: Removed Rope Arrows from this "never" list because there are a few places in later chapters where they can raise alarms because they are so close to a guard that the impact will raise suspicion.
  
  Edit 10-25-14: Removed Chests from this "never" list because I recently found one that, if left open, is eventually noticed by a patrolling guard, who becomes alarmed..
  
  Edit 11-13-14: Added Picking Pockets to the "never" list but there is one, and only one, exception. At the end of Chapter 7, in the Rotunda with the Thief Taker General. If you can manage to pick the General's pocket, for the collectable, without him knowing, you are penalized with an extra 15% OP over what you would have gotten if you had just escaped without doing anything. I don't know why that is. Edited 9-27-20: Thanks to Master Thief for verifying that picking the collectable from the TTG does not cause extra OP. (See his comment below, dated 9-23-20)

• A few examples of Opportunist Pie Chart Values (approximate)
  
  • Picking Locks (missed set point is heard) causes 6% OP.
    
  • Snuffing out a Candle causes 7% OP.
    
  • Dousing an open fire causes 8% OP.
    
  • Certain doors left open and noticed cause 18% OP.
    
  • Felling a hanging crate, bundle of lumber or chandellier gives 83% OP.
  
  Note: Some Opportunist Points are cumulative. If you snuff out 2 different candles, you'll get 7% for each one, for a total of 14%. Or, if you use a switch to turn out lights, and 6 hostiles notice, you will get 6X the individual OP.
  
  Percentages were incurred by individual actions alone. When other actions contribute more OP or Predator points, the combined points will change the overall percentages, e.g. if you start a chapter and dropped a crate, you would have 83% OP, at that time. Then, if you kill every hostile you see from there on, you will have amassed such a large number of Predator points that the OP would be reduced to a much lower percentage than the original value.

*I collected data from all 8 chapters and, all were played with Master difficulty level. I established a "control" setup by playing each chapter completely through, without grabbing any loot, collectables, documents or resources. I did the absolute minimum required to complete the chapter. Then, I would play the chapter again, doing only one thing, then completing the chapter, without doing anything else, and comparing the end rating with the control playthrough.

Control playthrough ratings...
• Ch 1 = 100% Ghost
  
• Ch 2 = 100% Ghost
  
• Ch 3 = 100% Ghost
  
• Ch 4 = 100% Ghost
  
• Ch 5 = 77% Ghost *
  
• Ch 6 = 100% Ghost
  
• Ch 7 = 86% Ghost
  
• Ch 8 = 100% Ghost

* Chapter 5: The Foresaken, is the only chapter where OP can be reversed. There are 3 lights that, if left alone, will turn off by themselves. If you turn them off first, then this chapter is 100% Ghostable.

Disclaimer - Obviously, I can't test every action in every area in every chapter. Knowing the game's penchant for inconsistancy, what is true for most circumstances may not hold for some other situations. So, the best I can offer is to say, use this information as "guidelines" rather than actual "rules".

I welcome any comments or suggestions.