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.