In the case of your example, rolling a 1 or 20 doesn't mean any more than rolling a 2 or a 19 or a 3 or an 18. In 3.5 at least, there are no automatic successes or failures with skill checks. Regardless, in most rolls there is not an automatic success or failure. Sure, if they roll a 20 they're going to be confident that they have succeeded. But that's no different than in real life. Sometimes you're absolutely certain of something (I rolled a 20), and sometimes you have no idea (I rolled a 1). That doesn't mean I'm right. And neither should your players always be right. Yea, your paladin rolls a 20, he's pretty confident. Still doesn't mean he's right. And on those kinds of checks, I absolutely never tell my players if they succeeded or failed for certain.

• "You're fairly confident that door isn't trapped"
• "You don't have any idea if that monster intends to eat you"
• "You're certain that the pit trap has been disabled. Oops, you were wrong."

OK, I don't have time to answer this as I want to. My background is in psychology, and I fell into role playing games when I turned 10 in 1976. So by the time I was in college, understanding where the term Roleplaying game really came from, I understood the critical nature of immersion, how it is the most important ingredient for game success.

And to be clear, the definition of immersion is to "Immerse oneself into the identity and Role of the part one is playing. To respond, as much as possible, as the person one is playing, not as oneself."

And before getting into the smaller details, I will dive right into the fact that the very system/game one chooses has a huge amount to do with the amount of Immersion.

Metagaming is the opposite of immersion. You use both terms, but I need to make that absolute definition from the beginning. This also means rules that encourage metagaming decrease the immersion in a game and therefore, decrease the main ingredient of a roleplaying game. The mechanics are called "Dissociated Mechanics", a term coined by Justin Alexander. This is very worth reading, because it gets into many of the larger picture issues with players being able to use in-game logic to see the world around them, as opposed to the rules forcing dissociation from in-game logic.

Once the players assume that rules are going to determine the content of an encounter or treasure (based on EL, or whatever) instead of what the environment or history of the area dictate, verisimilitude is lost.

Vreeg's Rules of Setting design are also heavily immersion related. My current campaign is 26 or so years old (started in '83). Building verisimilitude is a huge part of this.

• Vreeg's first Rule of Setting Design
  Make sure the ruleset you are using matches the setting and game you want to play, because the setting and game WILL eventually match the system.

  • Corollary to Vreeg's First Rule
    The proportion of rules given to a certain dimension of an RPG partially dictate what kind of game the rules will create. If 80% of the rulebook is written about thieves and the underworld, the game that is meant for is thieving. If 80% of the mechanics are based on combat, the game will revolve around combat.

    • Multiply this by 10 if the reward system is based in the same area as the preponderance of rules.

  • 2nd Corollary
    Character growth is the greatest reinforcer. The synthesis of pride in achievement with improvement in the character provides over 50% of the reinforcement in playing the game. Rules that involve these factors are the most powerful in the game.

• Vreeg’s Second Rule of Setting Design
  Consistency is the Handmaiden of Immersion and Verisimilitude. Keep good notes, and spend a little time after every creation to ‘connect the dots’. If you create a foodstuff or drink, make sure you note whether the bars or inns the players frequent stock it. Is it made locally, or is it imported? If so, where from? If locally made, is it exported?

• Vreeg's Third Rule of Setting Design
  The World In Motion is critical for Immersion, so create 'event chains' that happen at all levels of design. The players need to feel like things will happen with or without them; they need to feel like they can affect the outcome, but event-chains need velocity, not just speed.

• Vreeg's Fourth Rule of Setting Design
  Create motivated events and NPCs, this will invariably create motivated PCs. Things are not just happening, they happen because they matter to people (NPCs). There is no need to overact, just make sure that the settings and event-chains are motivated and that the PCs feel this.

• Vreeg's Fifth Rule of Setting Design
  The Illusion of Preparedness is critical for immersion; allowing the players to see where things are improvised or changed reminds them to think outside the setting, removing them forcibly from immersion. Whenever the players can see the hand of the GM - even when the GM needs to change things in their favor - it removes them from the immersed position. (Cole, of the RPGsite, gets credit for the term).

Remember that part of immersion is the lack of feeling walls around and rails under the characters. This means that the players should not feel that there are things that their character cannot do solely because of the rules or the GM's mindset. The job of the GM is to enable roleplay, not to inhibit it. This also means the GM must be as immersed as the players, or more.

Another big-picture thing that may irk some folk who sell stuff is that published settings can hurt immersion. They don't destroy it; but when the players have a lot of knowledge about a setting that their character would not have, this increases the opportunity to use it, consciously or unconsciously. Similarly, if your setting has its own bestiary that the characters learn as they go along, or at least a lot of homebrew tweaks, the players get used to working with the in-house data and not trusting the published sources.

If you have done all of this larger-scope stuff, the smaller scope stuff becomes easier. As a GM with miles on the tires, I find that playing up the level of knowledge my NPCs might have and do not have helps keep the players in the same mindset. Players key heavily off the way the GM plays their NPCs. They won't do the funny voices or the mannerisms if the GM does not, and if the GM is particularly careful about what their NPCs know and don't know, especially verbally, the players emulate this.