Background
I'm the DM of a role playing group. Improvised rule set.
I've created a world for my players where I have tried to be as close to 16th century reality as possible, with the additions of magic and phantasmal beasts being extremely rare and stigmatized to even talk about among commoners.
The characters
Two characters are present, a third was playing but had to pause due to becoming a daddy.
Here is a good place to mention that I encouraged my players to write a public story and a private story. There is one part of their background that no one else in the party knows.
First we have an assassin posing as a priestess. Her priestess side was supposed to be a cover, but she has now noticed that she has to actually play an inept priestess 90% of the time and the assassin she wanted to play only gets to come out when she sneaks away from her partner and gets a private session.
In order for her cover to be believable she cannot show that she can stand on her own in a fight, not that she can much, as she is specialized in poisons and sneaking, not combat.
The second character is the son of a jeweler who got down on his luck, lost too much money by gambling and was disowned by his father. Now he lives his life as a vagabond and survives by being very good at bluffing and sleight of hand tricks. He is decent at throwing knives but that does not help him in a stand up fight much.
The problem
In a realistic world, where the main characters are encouraged to do what is really in their characters to do, meaning for the most part, stay out of trouble and be cautious about strange things. How to get the characters to be passionate about going on a perilous, life threatening journey?
What I tried
- Mystical robed figures transporting strong boxes
- A Family on the run from assassins
- A wealthy guild of merchants ripe for a heist
I've told them (per realistic game system expectations) that I will enjoy killing their characters if they do things that are out of character. I've also told them about my trouble of getting them to be curious about things. We all agreed we want to stick to playing realistically though.
The passiveness of the players remains, I could hardly get them to save a tavern owner from local thugs without the help of some NPC muscle.
TLDR Question
What's a good way to motivate realistic, overcautious characters to risk their lives?
Best Answer
I would strongly suggest you to use famous Same Page Tool. Sit together, read the questions, discuss them. Now it looks like you all are trying to play different games, and no one is happy about it.
Your assassin has no goal, no connections, and why would she ever stick with someone she doesn't trust enough to reveal herself? Jeweler's son has no goal, but it seems his player at least isn't so disappointed. But what did he wanted to play when he rolled his character? It seems you don't know. You need to talk with your players and get to know such things. Your players need to talk with you and know what kind of campaign you have in mind.
Don't be shy to make some retcons or start again
It's your fun. So once you all know what kind of fun do you want to have, and made sure you are on the same page (using the tool or not), make it so. Don't stick to the game that's failing you.
Don't send mixed signals
If you allowed characters that would not be willing to investigate mystic figures with boxes, and you told your players that characters acting out of character will die, don't put mystic figures with boxes on your stage. This looks like GM's excuse to kill a character, from a player's point of view.
Yesterday I had to tell one of my players "Sorry, but that character wouldn't survive the campaign I want to DM. I can tell you why, but I'd rather avoid spoilers." And another player heard "Sorry, but in this campaign I don't think you would really have a way to benefit from this." These statements opened discussion and in the result players have characters that will still be fun to them - and I'm going to have campaign I'll be able to narrate, and have fun doing it.
People risk their lives for four basic reasons (and one extra):
They are paid to do it. Like, your assassin's guild gives you a target, or you are a mercenary. Or private investigator. Or king's investigator.
It's their social duty. It was every knight's duty to defend those who can't defend themselves. It is a noble's duty to know what happens on his land.
For their family and loved ones be it blackmail or sister in trouble
Because they will die anyway if they don't
Or there is just something with their heads. Think adrenaline junkies, people with mental health issues, homicidal maniacs. It does happen.
You just need to make sure your characters can have these motivations, and that you could use them to give them common goal.