Units are given faces randomly. If you wish to give a unit a particular face, use the setFace command. A unit's face can be changed at any point in the mission. If you wish to give a unit a particular face you need to add this code to the description.ext.
class CfgIdentities {
class John_Doe {
name = "John Bartholemew Doe";
face = "Face20";
glasses = "None";
speaker = "Dan";
pitch = 1.1;
};
};
Notice that there are no spaces in the class name John_Doe. You can then set a unit's identity using the setIdentity command.
A list of all OFP default faces can be found here.
A list of all Arma default faces can be found here.