The following is an excerpt from page 3 of the Warhammer Annual 2002 release with an expanded explanation by Gav Thorpe regarding Characters movement while mounted or on foot.

You'll also notice that the rules make characters on foot a lot more flexible than those mounted on monsters or other beasts. Again, this was a deliberate change to make players think twice about sticking their Wizard or General on a steed or monster. Not only does this restore some of the balance between regiments and characters, but weighing the pros and cons of being mounted or being on foot makes selecting your army more of a challenge.

In all movement and line of sight respects, a man-sized character on foot should be thought of as a lone skirmishing model - able to march freely, charge and shoot in any direction, and so on. This means that, until engaged in combat, a lone character on foot does not count as having a flank or rear zone - the character can freely turn to face the charging enemy. Once fighting, however, they are locked in place and can be charged in the flank or rear as normal. Also note that although characters on foot (and skirmishers for that matter) can move at double their normal rate for movement, this does not mean that they move at double rate for a failed charge. In this case their 'normal movement' is considered to be the distance shown as their Movement characteristic (more cunning devils at work out there!).