How much do you tell the players in advance?Edit
It’s important that although you shouldn’t tell them everything, your briefing should tell the players enough to let them design suitable characters. Most of the time, that’s not an issue: if you run a game based on Blake’s 7'™, say so and the players will know all they need to – space opera, bad-guy Federation, players will be rebels, need skills to operate starships and guns that go ‘zzzap!’
But sometimes you will want to hold something back to surprise the players as their characters are surprised. You must make sure that their briefing doesn’t cause players to invest too heavily in ‘unsuitable’ skills. One example was when a GM asked the group I played in to design characters which were “based on yourselves”, and she wanted at least one character to work for MI5. As I work with computers for the Civil Service, my character became an MI5 cryptographer and I invested over 25% of my character points in appropriate computer, electronic, and advanced maths skills. The campaign then turned out to be a ‘Quantum Leap’-style uncontrolled time-travel game. We never adventured later than 1940 – my character was effectively ‘worth’ 25% less than all the others.
The GM knew in advance that we would never be in contact with the technology of the 21st century, and should have made sure that the characters fitted the campaign. As a GM in a situation like that, I would have taken the player aside when reviewing their character and said that the party should be in touch with MI5 often enough to hand the computer stuff over to others. The player should keep a little skill for basic computer operations, but expect to be more of a field agent and hand off the complex hacking to the experts.