Targett, a member of Australia's Olympic 100 metres freestyle team, knows the two intimately.
The towering 26-year-old shared a house with Brazilian Cielo while swimming at Auburn University in the United States and remains good friends with the Olympic 50 metres freestyle champion.
Targett now trains intensively with Magnussen, the heavy favourite for the individual 100 freestyle gold in London, as they bid to repeat their relay gold from the Shanghai world championships at the Games.
"It's hard to say why (Magnussen) has such confidence, but it's really impressive," Targett said.
"The only other time I've seen it in another athlete is Cesar Cielo, who I went to college with.
"So it's really exciting to see because I know his mentality. I can sort of understand his psychology because I've seen it over and over again, especially with Cesar in 2008 and 2009 and that's when he was at his peak."
Australia's imperious record in the Olympic pool has created a tidal wave of expectations for the country's top swimmers that crests every four years and occasionally wipes out the hot favourites.
Eamon Sullivan marched into the final of the 100 freestyle at the Beijing Games after smashing the world record in the semi-final but finished with silver after being trumped by French great Alain Bernard.
With precious few titles expected from Australia's Olympic swim team, rated the weakest in over a decade following the retirements of greats such as Ian Thorpe and distance swimmer Grant Hackett, Magnussen bears the brunt of the Australian public's hopes in London.
Targett, who took silver in the 4x100 medley at Beijing, believes a Magnussen meltdown is highly unlikely given the laid-back swimmer will have the 100 relay to shake off his nerves on day two of the Olympic swimming tournament on July 28.
The individual event runs from July 31-August 1.
"I don't think this pressure will get to James at all, thankfully, because our relay is on the second day," said the English-born Targett, who also won bronze with Sullivan in the 100 freestyle relay at Beijing.
"If he were to crumble it would be then, but I don't see it happening."
Magnussen swam a scintillating 47.10 seconds at national Olympic trials in March, only 0.19 short of Cielo's world record of 46.91 set at the swimsuit-tainted 2009 world championships.
Apart from Cielo, only Bernard (46.94) and Sullivan (47.05) have swum faster and none since polyurethane suits were banned.
A raft of swimming pundits - and Magnussen, himself - believe he can improve on Cielo's record at London, but Targett said the Australian was unlikely to have the world mark high on his priority list lest it distract from his ultimate goal.
"I think he'll definitely go faster at London than the Olympic trials, but there's still a couple of tenths (of a second) there," said Targett.
"I know his number one goal is to win the race and the world record is secondary. To come home with an Olympic gold medal is an unbelievable achievement in itself."
With Magnussen the lead-off swimmer in the freestyle relay and the other five swimmers in the team blitzing the Olympic qualifying mark at the trials, Targett has every chance of adding gold to his Olympic silver and bronze.
But he expects world championship runners-up France and bronze medallists, the United States, to mount a fierce challenge in London after being upset in the Shanghai pool.
"I don't think (we're unbeatable)," said Targett. "I do think we have tremendous depth and we are the team to beat.
"The level of competition and depth we have is keeping everybody honest. But I think our four guys, we have to be the four fastest in the world, otherwise who knows what could happen on the day."