A character model 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 if they get shot!
A character model within 5" of a friendly unit of five or more models can only be picked out as a target if he is the closest possible target. This restriction enables characters to move around behind the battlelines without attracting an unrealistic and unreasonable amount of missile fire.
If a character is part of a unit which consists of at least five models in total then he cannot be shot at. Any shots against the unit will hit ordinary troopers and not the character. If the unit drops in size to less than five models, then further hits are randomised in a suitable way. For example, in the case of a unit consisting 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. In this case 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.