This feature is from my MOD - Everyone Can Be The Leader.
http://forums.taleworlds.net/index.php/topic,46178.0.html

As the title suggests, this feature will allow you (the player) to pick a hero (this includes your character of course) to control in battle so that they fare better in combat and can upgrade easily. As a bonus, this means that as long as there is a NPC who is not wounded, you do not have to resort to auto-resolving fights (as anyone who has played M&B knows, auto-resolve is not exactly the best way to finish fights...).

List of changes:

module_game_menus: (20 places)
*the menu options of selecting the commander for any applicable mission
*the menu of selecting a companion as the commander
*check whether the selected commander is wounded for any applicable mission
*judge the sex of the selected commander to decide the picture at the bottom-right in the background
*codes make the player regain hp lost during mission according to the skill level of first aid when the player was not the commander
(the skill first aid take no effects to the player when the player was not the commander)

module_mission_templates: (9 places)
*check whether the player can join battle when the player was not the commander
*display messages about the changes of npc's proficiency when he/she was selected as the commander

module_scripts: (5 places)
*some changes of battle statistics and mission casualties

module_triggers: (1 place)
*reset the commander as the player when back to the map

Credits:
rubik - Originator
fisheye - Some codes from autoloot(the structure of the menu for selecting commander)
oolonglgx - Suggestions