We have some neat threads going on right now which are touching on how a feeling of "realness" can emerge from play where there are some facts about the fictional world that are both (a) true and (b) not known to all (or possibly even any) participants.
One example that's been mentioned is the random table, where the contents of table have established a possibility space, and the procedures that invoke the table can "reveal" the "truth" of one item from that space.
I don't think I agree with that feeling "real". At least, not when it happens in the moment, as needed.
Obviously it's very efficient and practical to establish game facts when we need to know them, so we don't waste a lot of time establishing game facts in advance that we'll never need to know.
And I don't think it's all that tall an order for a player to take content that's generated on the fly, and deliberately invest in treating it as real.
However, if we're talking about aspects of games themselves, not just of players, which help a game feel real, I don't think this qualifies:
Rounding a dark fictional corner in character and then consulting a piece of paper and some dice at the table to see what's around that corner.
This doesn't feel real because the at-the-table causal chain of fiction-creation is just too obvious. To feel real, ideally we can believe that the orcs around that corner were already there as a true game fact before we decided to round that corner.
Having said all that, I don't want the GM to have to prep what's around every corner in advance (although that is exactly what the most real-feel-enabling GM I've played with does) because that's a ton of work. And I don't want the group to have to flip through a giant book that lists what's around every corner, because hunting down the amount of info you need in a non-railroaded RPG gets distracting and time consuming (and expensive to acquire).
My preferred solution to all this is to randomize whatever needs to be randomized before* it's encountered by the characters, and to give the GM (or whoever's revealing game facts to the other participants) an orientation to the fiction which allows them to:
a) ideally, feel like they're discovering game facts as they invent them, but if not that, then at least:
b) invent game facts based on reference to the existing truths of the game world, as opposed to mere personal whim.
If I am GMing a game where I know that caves like this are often filled with orcs, and the players don't know this, then even if I hadn't planned for an orc to be around this corner, when the players announce their intention to round this corner, "there are orcs" jumps naturally to my brain and can be delivered instantly to the players in a process indistinguishable from communicating pre-established truth.
I think this is why, over the history of roleplay, so many realism-seeking GMs have bought and read and enjoyed so many setting facts. Once the book makes the world feel real to you, you can make it feel real to the players.
*Note: this needn't be done before a session; this can be done during a bathroom break after the players have just announced they're going to go somewhere the GM doesn't have a great feel for.
I'd love to hear others' takes, and especially techniques for getting that "real" feeling.