As you begin to progress through the campaign, all you have left are reserve operatives. If you do, read on. Read through the operatives dossiers. You will find that there are certain guys that are more stealthy than others. Use them on the infiltration and phone-bugging missions.
Some shooters are better than others.
If you are likely to be sniping, make Mr. Accurate the team lead. Don't choose a Demo expert to lead an embassy breach, likewise leave the snipers behind when the team is tasked to clear a number of rooms. Tangos could be anywhere. Choose a weapon with a high pulse-rate, and a heavy punch. Choose a powerful pistol and heartbeat sensor as your backup plan.