The tendency for RPG sessions to "devolve" into humor and zaniness, even when a different tone is the goal.
Fun-time. It's a game you play for fun in your leisure time. Often you are tired and/or punchy. Guarding yourself is something you do all day and when you are among friends you may tend to "let down."
Nerves. Roleplaying is like skinny dipping; it's not a spectator sport. Role playing requires that you concentrate on playing "through" your imagination. This puts you in a liminal space that exposes you and means you may have less of a filter on what enters your mind and/or comes out in the moment. Nervousness about being thus exposed can cause people to distance themselves by joking or tittering out as a knee-jerk reaction.
Tropes. We often draw on images from literature, movies, videogames etc. These associations are powerful but can lead to further associations. A seriously toned "space knight" game can remind someone of Star Wars, and then Space Balls, and then "I see your schwartz is as big as mine!" comes out of their mouth.
Get people relaxed and comfortable with the tone, goals, session.
Foreground your source material.
Try to make the meta situation approximate the in-game fiction.
I ramble on about my ideas for having gods walking the earth and what iron means in a setting where the technology is still new. Mostly this is tied to my Gygax 75 Challenge setting: Uzrak, Where the Gods Walk. You can read about it at the Viridian Scroll.
Che Webster of the Roleplay Rescue podcast asks the question, what's the difference? If you make a consistent ruling, isn't it then a rule?
In my opinion, No. A ruling is a practice at the table. A rule is a prescribed process in the book.
Rules are made 'in a vacuum,' they are less flexible, and if there are too many of them, they are hard to remember. OTOH, rules are impartial and transparent. Rules should ideally (my opinion) address only the most common situations.
Rulings come from rules, but should never (or at least rarely) become rules. Rulings are made at the table, require buy-in from the group (and therefore generate investment), and are flexible. They can evolve over time like language. OTOH, rulings can flex so much that they feel inconsistent or even unfair.
That's the basics of what I say in the cast. :) Now you don't really need to listen, though I try to illustrate with examples.
I talk about how Dave Arneson worked as a disruptive player to be a catalyst in transforming wargames into role playing games. I also talk about challenging players and players who challenge us to change, including players that represent diverse opinions and experiences.