FLC PROCEEDED DIRECT; INSTEAD OF MAINTAINING ASSIGNED HDG.

Narrative

I REMEMBER THE CTLR STATING; 'TURN R 120 DEGS; VECTORS TO THERMAL' AND BOTH THE FO AND MYSELF (THIS WAS INITIAL IOE FOR FO ON THE SO PANEL FOR LAST 3 YRS). A MINUTE LATER; THOUGHT THE CTLR SAID 'PROCEED TO THERMAL WHEN ABLE.' AT THE SAME TIME OF THIS TURN/CLB; A ROGUE SCREEN APPEARED ON THE FMC LEGS PAGE WHICH DIVERTED OUR ATTN. IT WAS RELEVANT BECAUSE THE FO WAS TRYING TO SELECT THE 'LEGS' PAGE SO HE COULD ENGAGE THE LNAV COMPUTER TO PROCEED DIRECT TO THERMAL. WHEN THE FO SUCCESSFULLY SELECTED THE LNAV DIRECT TO THERMAL; THE CTLR ASKED OUR HDG. WHEN WE TOLD HIM '095 DEGS' HE STATED WE WERE CLRED HDG 120 DEGS. WE BOTH THOUGHT WE HAD BEEN CLRED DIRECT TO THERMAL. THE FO WAS ENGROSSED IN COMPLETING THE TURN TO THERMAL AFTER QUESTIONING THE APPEARANCE (UNCOMMANDED) OF AN UNUSUAL TECHNICAL VISUAL SCREEN ON HIS SIDE. SHORTLY THEREAFTER; THE CTLR CLRED US AGAIN DIRECT TO THERMAL. IF WE MISINTERPRETED HIS INSTRUCTIONS; IT WAS BECAUSE OF THE NEWNESS (FIRST FLT SERIES) OF THE FO IN THE COPLT'S SEAT AND MINE FOR NOT LISTENING INTENTLY ENOUGH AND NOT SLOWING DOWN THE FO'S NIMBLE FINGERS ON THE FMC. NO OTHER ACFT WERE SIGHTED DURING THIS PERIOD IN VMC.