Often it has been asked of boardgame designers whether they start with mechanic or theme, and there never has been a decisive answer, since it seems that often they don't know themselves which they might have come up with first, and it is not always the same.
Perhaps they were pondering a mechanic, like deck-building or worker placement, when a thematic idea struck them and they knew how the mechanic would fit the theme, or perhaps the other way around. Chances are that many start with both in their heads until something connects with something else.
Some, though, and I am guilty of having done this before, get so wrapped up in the theme, adding in details as they think of them, that they forget about the purpose of a game. It is through this that we get historical reenactments, where any deviation from actual history is either punished or not accounted for in the game. My own was to fit so much cricket information onto two poor, overworked slabs of cardboard that I eventually just went out and played the sport instead.
Perhaps the best answer I have heard about starting with theme is to find out what is fun about a topic - ANY topic - and replicate that experience at the table. And I agree.
But I'm going to go Hitchcockian on you.
Alfred Hitchcock started with a dilemma, somewhere around the end of the first act. He wanted to present an awful choice that the characters must make, then have the audience reel and squirm as they came closer to making it. The spy whose job it is to start a relationship with someone, only to be told to then betray her. What does he do? What would the audience do if they were he? Love triangles, power exchanges, desperate acts, betrayal of duty all abounded in Hitchcock's world.
Theme was not important. Plot was. Plot gets you to the decision. Hitchcock pointed out that theme was a McGuffin - an object that the characters cared a lot about, but the audience didn't, so don't bother explaining too much about it except that it is there.
The whole film, then, became about conveying plot. Presenting only the information necessary to get the audience to understand the elements behind the choice he is about to make, and the rest of the film is the resolution of that decision, with decision upon decision being made after that either to carry out the plan decided upon or to rescue himself from the trouble that that decision has caused. Usually both.
Games, then, should do the same. Start with the decision you would like your players to make. Do you want them to bargain? To trust each other? To betray? To build, attack, liaise or gamble. And we ought to get to this second act as soon as we can, without additional information.
The theme can quite easily be given by the box itself, or the first page of the manual, or as a scenario is read out, but while the theme is fun to help players' imaginations get involved, they will be listening out for plot. What do I need to know to make decisions in this game? So there are 15 turns until sun-up and we have to survive - do we ALL have to survive? Are there enough resources to go around, or are there ways of getting more? What does my character have to do to ensure his own survival? As tiles are laid out, players look for plot-driven opportunities. As cards are gathered along the way, this affects their choice of direction in the game.
Next time you play a game with character cards, read it for how it affects plot, as opposed to theme. Some might merely describe the character's physical appearance, age, height or background, but unless these alter how you will use that character to make decisions later, it is merely theme. Cosmic Encounter is on many people's list of game favourites, and apart from its simplicity, it is largely due to the character cards which alter each player's plot so greatly that everyone is on their own path trying to use their own race's abilities to best advantage. The plot of character against character becomes paramount. The theme of space is merely there to give such a range of races to invent.
We have recently seen the secret character cards and agendas coming back into games. Perhaps the best example of a plot base with theme I have seen is Dead Of Winter.
Dead Of Winter - the theme is on the box and on every piece of artwork, and not much else needs to be said! You are in a frozen town, supplies are running low, you can't move easily, and zombies are barging in. The atmosphere is claustrophobic already!
But now plot comes in. Scenario. What do we all have to do to make it through this? Now I get my secret agenda. What decisions do I have to make to personally win? Now character cards - and already I have a choice which will affect the plot. The characters have abilities useful for the scenario and secret agenda. Once I have chosen, this will also affect the decisions I will make later. Do I choose that which will help the group, or myself? I haven't even started playing yet and the game is playing me!
Is that plot enough? No! Every round there is another plot crisis, every single turn are 'crossroad' cards. Theme, PLOT! Theme, PLOT! Throughout the game I make many, many morally ambiguous decisions, and I can't tell anyone my reasoning because some of it is my secret agenda! I alone own my personal dilemma, much like I were a character in a Hitchcock suspense drama.
Not that all games can nor should be so heavily plot-driven, but it pays to know the difference between plot and theme and, when embarking on an idea for a game, start with the dilemma you want them to have, work backwards from there, and waste no time in getting them there.