If a character is travelling with a unit of at least five other models of comparable size to himself or larger, then he may not be picked out as a specific target by enemy bowmen or other shooters. In reality it is difficult amidst the confusion of battle to single out a target in this way, so any shots at the unit are worked out against the ordinary troopers and not the character. If the unit drops to less than five models then hits may be randomised in any suitable way. For example, in the case of two ordinary models and a character there is a 1 in 3 chance of hitting the character so roll a D6 for each hit scored. Any roll of 1, 2, 3 or 4 shows an ordinary model has been hit; a roll of 5 or 6 indicates the character is hit.
If a character model is within 5" of a unit of five or more models of comparable size to himself, then he may only be picked out as a target by enemy shooters if he is the closest target. This means that characters can move around behind the battlelines without attracting an unrealistic and unreasonable amount of missile fire.
Character models more than 5" from a friendly unit of five or more models can be shot at without restriction. Characters prone to jumping up and down in front of the enemy are asking for it, so it serves them right!
When you are deliberately shooting at a character model at any time there is always an additional -1 to hit penalty. This is because the normal chance of hitting assumes the target is massed up in ranks, or is pretty large to start with. This is not the case when you are shooting at a single man, especially if there are other potential targets to distract the shooter's attention.
Note that if a character rides a horse, or a large monster, then he is a larger target than he would be on foot and won't be able to hide among a unit of troops on foot. Such characters benefit from additional saving throws in the case of cavalry, or the chance of a monster being hit rather than its rider in the case of large monsters.