I haven't played this game, but the rules seem fairly clear to me.
1) You can swap which character is covered, and which is exposed, each game turn. So they don't have to stay in the background.
At the beginning of any turn, after dealing with such cards as Jail
and Dynamite and before Phase 1, the active player chooses one
character to be exposed and the other to stay cover. In order to do
this, push the exposed character towards the center.
2) You only get to use your exposed character's ability. The exception is when your covered character is targeted, in which case you get to make a response from them instead (poking their head out of cover).
During your turn, or your opponent’s turn,
you can only use the ability of your exposed character. However, if
your covered character is the target of a card (BANG!, Indians!, Duel,
etc...) you temporarily get the ability of your character in cover
(and lose the ability of your exposed one), for the short time needed
to respond (see example below).
3) Your hand size is determined by the life remaining of your exposed character.
When you declare the end of your turn, you must discard from your hand any
card exceeding the current life points of your exposed character.