Murphy and Castrogiovanni may be team-mates at Leicester and partners in a restaurant, however they will be enemies for 80 minutes in Dunedin.
And with Castrogiovanni having earned a reputation as one of the world's top props, Murphy knows Ireland may have their hands full.
But he believes he has the perfect tactic to put his great friend off his game.
"I'm big into my scrummaging, so I'll definitely be telling my forwards about how to scrummage him!" he said.
"I'm thinking of leaving some baskets of chips in the corners of the changing room for him. That should do it.
"The amount he eats, it should distract him quite nicely."
Murphy and Castrogiovanni have been in regular contact throughout the competition, although the volume of messages has increased in recent days.
"He's a good mate of mine, he's a fantastic prop and he's a different character off the field to what he is on it," he said. "He's certainly one of Italy's key players.
"He's been texting me. The closer the game gets, I'm receiving more and more text messages from him.
"He swears a lot at me in Italian and he's trying to teach me all the bad words. I'm not going to repeat them."
Sunday's showdown will determine who progresses to the quarter-finals out of the Pool C rivals.
Ireland are 1/10 favourites to prevail after amassing 15 successive victories in the fixture, but Italy were denied only by a late drop-goal in their last meeting in February.
"It's the biggest game of the pool," said Murphy.
"After they lost to Australia, Italy said they're targeting this game.
"They've openly said they want to reach the quarter-finals, which basically means they're saying they want to beat Ireland.
"They've improved massively over the years. They're a serious side. They're a very physical side but they're playing some good football alongside that now.
"Some of the tries they're scoring at the World Cup have been good tries. Our backs will be against the wall as much as theirs will.
"It's Italy, but they'll raise their intensity from the Six Nations. It's the World Cup, it's a level up from the Six Nations and I'm sure the game will reflect that."
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