(Metal Gear Solid V: The Phantom Pain) How to play as other characters?

I've unlocked two characters you can play as during the game (the first guy and the woman) but whenever I try to select a different character, nothing shows up with the exception of Big Boss. Do I have to wait a little longer and keep playing the main story, or is there other requirements?

Thanks in advance.

@DuskStrider: Only characters in your combat unit can be used for deployments, afaik.

You need to be able to have the option to create a combat unit build the combat unit platform. Assigning staff for that unit will enable them to be playable. It's the stats with the two swords crossed in the character stat sheet.

Cant remember what mission it is when it unlocks, but just keep going through them until you