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Primarily, Motivations (like beliefs from Burning Wheel) in my game are used to drive goal setting or complicate things in interesting ways for the character.

But Im having a hard time backing that mechanically, particularly for the 'complicating' aspect.

The way I see it, having a mechanic that rewards acting on a motivation means people will hammer the button to get the food.  But one of the interesting things about roleplaying is deciding under what circumstances your character will act on or break with their motivations.  So rewarding only 'acting on' defeats that somewhat.

What Ive done is to reward either option - simply narrating considering the motivation during a decision is enough to get a reward. Which is explicitly what Id like the players to do, but the rule itself is kinda wishy-washy.  It doesnt have a solid edge to it if you know what I mean.  And its hard to get that edge because I dont want the player to make the decisions tactically based on a mechanical trade-off, I want them to make the decision according to the fiction that seem right to them.

Can you give something like this solid mechanical backing without destroying the waht you are trying to back up in the first place?

If so , how?

Hope this helps:

I've been using the Aspects mechanics from the FUDGE system for that, though in a limited way. I made a list of aspects that can be invoked either by the player to get out of trouble or by the GM to get players in trouble. Players choose three on character creation. When a player uses one aspect to get a second chance or a bonus to his roll, he spends a story point. When the GM invokes it, he gives the player an story point. Depending on how difficult you want this mechanic to be for players, you can either rule that they can reject the story point to avoid the GM fiat in this case, that they have to pay a point to negate the GM fiat or that player's got no choice about it.

But the thing is that the character motivations must be something that brings complications all the time: For example "I fight for justice" should get players into trouble each time it's invoked, otherwise it won't affect the story. It doesn't have to be always a positive aspect. I used something like "shoot fist, ask questions later" and invoke those aspects on a player whenever suspicion arised over any situation.

hi.  That mechanic is different in nature to what Im looking at.  It seems to me like it deals with traits affecting consequences whereas Im focusing on traits affecting decisions.  would you agree?

Hmmm well, perhaps it's about the degree at which the player decision is influenced by the trait. I admit i'm using Aspects like a Alignment mechanic, but instead of making it too general like "chaotic good" I apply it over an specific character reaction. Of course, I give players the choice of ignoring the Aspect invoked, but most of times they agree that's the way their characters would react.

So perhaps the way I'm using aspects may be putting a limit to players reactions, and that's why it affects consequences instead of decisions. Now, I could have tried some more general motivations, but at least for my group of players I found that the ones which worked the best were the ones that got them in trouble, complicating the story and making it more interesting. I found that adventurers are the kind of heroes that get in trouble because they look for it, and thus started to reward players who got in trouble because of their characters principles.

Perhaps it would help you to think backwards: what kind of players reactions do you want? How much an how do you want their decisions to complicate the story? Will your players do that by themselves or will you need a built-in mechanic to make them jump into trouble?

I found my group of players kind of lazy, they will always try to do everything the easy way or remain indifferent in front of situations that doesn't involve them directly. So the Aspect mechanic became useful to make them jump into trouble and have them push the story foward. More collaborative players will only need motivations as guidelines to play their characters, I think.