ACR MAKES WRONG DIRECTION TURN AFTER CLRING RWY.

Narrative

WE WERE LNDG ON RWY 31C AT MIDWAY. I HAD BRIEFED THE APCH INCLUDING PROBABLE TAXI RTE AND WAS PLANNING A R TURN OFF RWY 31C. ON ROLL OUT THE TWR GAVE TAXI INSTRUCTIONS WHICH WERE 'R TURN AND BACK TAXI ON 13L'. LEFT STUCK IN MY MIND AND I THOUGHT I HEARD 'BACK TAXI ON 31L.' AS I TURNED L OFF THE RWY MY CO-PLT SAID HE HEARD A RIGHT TURN. I SAID I HEARD A L TURN AND HE SAID NO MORE. WHILE BACK TAXIING ON 13R/31L THE TWR SAID WE MADE THE WRONG TURN. THEY GAVE US FURTHER TAXI INSTRUCTIONS. SEVERAL THINGS COME TO MIND: TWRS NOW GIVE TAXI INSTRUCTIONS DURING BUSY TIMES. EXAMPLE: ON LNDG ROLL OF SHORT RWYS WITH OBSTACLES AT END OF RWY. MY CO-PLT WAS 3 MONTHS ON THE AIRPLANE; 3 YRS AT ANOTHER AIRLINE. HE USED 'SIR' ALL THE TIME WHICH I SHOULD HAVE DISCOURAGED. THE 'SIR' I FEEL CREATED MORE UNEQUALNESS. FROM CLR I KNOW ONE OF MY PROBLEMS IS WHEN SOMEONE QUESTIONS A DECISION IS DOESN'T ALWAYS RAISE A RED FLAG IN MY MIND TO SAY STOP; LET'S CHK THAT. THE ASSERTIVENESS OF MY CO-PLT PLAYS A BIG PART. I FEEL THE CO-PLT BEING SO NEW BLOCKED HIS NORMAL ASSERTIVENESS.