DELAYED START OF DSCNT.

Narrative

ZLA TOLD US TO DSND FROM FL350 TO FL280. I ANSWERED AND CHANGED THE ALT WINDOW (B-757) TO FL280. MY COMPANY'S PROC IS FOR THE PNF TO CHANGE THE ALT WINDOW AND TALK; WHILE THE PF WORK ALL OTHER MODE CTL PANEL CHANGES (WITH THE AUTOPLT ON). WE WERE GETTING READY TO DO A LYNXX-7 DSCNT INTO BUR WHEN ATC 'INTERRUPTED' US. WHEN I DIALED IN THE ALT AND ANSWERED ATC; I IMMEDIATELY WENT BACK TO WHAT I WAS DOING -- PROGRAMMING THE FMC FOR THE DSCNT; ASSUMING THE CAPT WOULD INITIATE THE DSCNT. AFTER A SHORT WHILE; THE CTLR SNAPPED AT US TO BEGIN THE DSCNT AND GAVE US AN ALT RESTRICTION TO MAKE; WHICH WE MADE WITHOUT A PROB. I DON'T THINK THERE WAS A SERIOUS PROB; BUT THERE COULD HAVE BEEN. WHAT BOTHERED ME WAS THAT; INSTEAD OF JUST DOING WHAT I'VE ALWAYS DONE IN MY FLYING CAREER WHEN I'M TOLD TO DSND; PULLING PWR AND DSNDING; THE FMC IS NOW MY FIRST THOUGHT WHEN I WANT TO MAKE THE ACFT DO SOMETHING. IT'S WONDERFUL MOST OF THE TIME; EXCEPT DURING TIME CRITICAL PHASES OF FLT WHEN IT CAN BECOME AN ATTN MAGNET.