No it doesn't all depend on what you work on -- at LEAST as much depends on personnel.
Pat Riley was a great defensive coach in his latter years. But its not because he sat around and made crappy defenders into great ones. He went out and GOT great defenders. His whole structure was built around great defensive centers who were great defensive centers BEFORE he got them (Ewing/Mourning). Then he went out and imported other top flight defenders, found them in camp, traded for them, traded away weak/middling guys to get ever more defense.
Gregg Popovich is a great defensive coach, but he has NEVER had to coach without great defensive interior people. David Robinson was a defensive monster long before Pop arrived. Tim Duncan only led the NCAA in blocked shots. GREAT defenders, not by practice, but by nature.
Larry Brown preaches defense, but where has he had his greatest success with it? Philadephia, where he rebuilt the team from the ground up for defense defense defense. Imported Theo Ratliff, to guard the paint, already a great shotblocker, then traded for Ditkembe Mutumbo, who had only won three(?) defensive player of the year awards. Then he goes to Detroit and finally wins a championship. And what does he have anchoring the paint? Only Ben Wallace, the defensive player of the year before Brown arrived, and then a trade for another excellent interior defender in Rasheed Wallace.
These coaches didn't make weak interior defenders into championship level defensive players, then either went out and intentionally imported that type of player, or they intentionally took over teams with that type of player already. Not one of those players ever became a great defensive player because of those coaches. They already were great defenders.
You know what Riley would do if he took over the Kings? Make trades. Brad would be gone in about a nanosecond if Riley could find anybody out there. No post game to build the offense around, no shotblocking = goodbye if you're a center in Riley's system. He wouldn't sit around and waste time trying to make Brad into something he wasn't. He'd just go get somebody who already was, just as all of those coaches always have. There's a reason Pop went out and got Rasho instead of Brad two years ago.