When deliberately shooting at a man-sized characcter model on foot there is a -1 to hit penalty, as described in the Shooting section. This is because the normal chance of hitting assumes that the target is massed up in ranks. This is not the case when you are shooting at a single character, especially if there are other potential targets to distract the shooter's attention.
Note that this -1 does not apply if you are shooting at a character who is riding a steed or a monster, as described in the Monsters section. In such a case, the shooter does not suffer the -1 penalty and benefits from the +1 to hit modifier if the monster ridden by a character is defined as a large target.