Some weapons such as stone throwers have an area template that the firer is allowed to place anywhere he wishes. This allows the player to deliberately aim his shot on a character model. Cannons also allow you to choose exactly where to aim shots, and so can be aimed directly against characters.
In the case of weapons which can be aimed in this way, there is a special rule which allows characters to either jump out of the way, throw themselves to the ground, or somehow avoid destruction by their amazing luck or uncanny reactions. This is intended to prevent characters becoming targets for these weapons in a manner which is unrealistic, unfair and definitely unheroic! This rule applies to all weapons which are not restricted by the normal rules regarding targeting characters as already described.
A character model who has joined a unit may be lucky enough to avoid death from cannon shot, a boulder from a stone thrower, or a comparable missile, thanks to a warning shouted by a comrade. Possibly one of his companions shoves him out of the way, or pushes him to the ground. A trooper spots the fateful missile heading towards his leader, and shouts 'Look Out Sir!' or some such warning, and hopefully alerts the character in time for him to duck or adroitly avoid the hurtling missile. The missile slips past the character and hits the man standing beside or behind him.
If a character is with a unit and is hit by cannon fire, a stone thrower, etc, then roll a D6. On the roll of 6 the character fails to hear the warning and is hit by the missile. Work out damage as normal. On the roll of 1 to 5 the character is alerted to the danger and avoids the missile. The character is not hit and the missile strikes another model instead, so transfer the hit onto an adjacent model in the unit.