I played in a Google+ Hangout game recently where players were taking big chances with their characters and justifying it with "this is only a one-shot." I want to state that I appreciate the actions there characters took, I really do. I wanted to know what the glowing sword and armor did. I wanted to know what was behind the black curtain. I did. And I'm glad they took the initiative to check, because I wasn't going to be the one checking it out.
There are two sides of the same Old School coin. One side says that characters are dispensable. You know they are going to die. The game is deadly. Don't get too attached. Be prepared to roll up another character. That's all good advice.
The other side of the same Old School coin says that you are going to be careful with the character. You know that the game is deadly, so you're going to minimize your risk by taking fewer chances. Not that you won't open the mystery door, but that you are going to look, listen, check for traps, check for runes, and they cautiously open the door when the time finally comes. By that point you have done everything within your power to minimize your character's risk of death.
That brings me to roleplaying, the title of the post. The people saying, "it's only a one-shot," and charging headfirst into danger aren't always playing the role they have selected.
I think it's safe to say that the majority of the characters we create, if asked outright, would admit that they really really really want to survive this adventure. They are not ready to die (or else they would have converted to the elfy religion before the adventure and given themselves some sort of certainty about their afterlife, but I digress with specifics from the Hangout game). These characters want to live! Yes, they know that death is possible. They have heard tales of the mortality rate of people who go into Undermountain (for instance). They have decided to go anyway, but they are likely going to try to minimize the chance of death.
So if I'm playing a game with you, and I'm not volunteering to touch the glowing item, or to search the magical darkness, or to barge into the room with no battle plan, I'm not trying to be difficult. I've actually gotten into my role, and I'm playing a character who wants to live.