Sexton has one Farrell in his corner and another in his crosshairs at Twickenham
Ireland out-half must be at his best to get better of familiar face in England No 10 shirt
Johnny Sexton has two similar photographs from the Lions tours in Australia and New Zealand in his home.
The first photo was taken after the final kicking practice on the eve of the third Test in Sydney in 2013. It's of Owen Farrell standing beside Sexton, kicking coach Neil Jenkins and Leigh Halfpenny.
Four years later in New Zealand, they replicated the picture the day before the third Test in Auckland.
Again, they all had their arms linked, smiling at the camera, Farrell and Sexton on the same side, part of a special group.
Farrell and Sexton are almost a replica of each other. They are two players - and also vice-captains of their countries - who are hard-wired with a similar drive, feistiness and intelligence.
They built a mutual appreciation from the '13 Lions Tour with Sexton calling Farrell an honorary Irishman as he hung out with the Irish players more than his English team-mates.
They were "joined at the hip" on last summer's Tour, according to Owen's dad Andy.
"They are constantly talking about rugby, they room together. They are very similar characters," Farrell said before the second Lions Test in Wellington which saw Sexton start at 10 and Farrell at 12.
"More than anything, it's how they make people feel, how they make the people around them feel; the energy they bring".
Seán O'Brien said the Lions attack only got better when Sexton and Farrell ran it in the build-up to that victorious second Test.
Sexton tried to play down O'Brien's words but he still showed how influential the pair were: "Owen drove things early when he was number 10 and then I came in and we drove it together. It wasn't just player power."
That's the kind of player power any head coach would want. Sexton and Farrell were the mini dream team in that Lions squad.
After Sexton came through form and confidence questions in relation to his pecking order with Farrell, the Lions management finally went with the formula everyone else had figured out months previously.
When Sexton and Farrell played the 10-12 partnership for the first time against the Crusaders, they had to have an implicit trust in each other, knowing they'll only be as good as they make each other look.
"At times, he was at 10 and I was able to be his eyes, and at other times, I was at 10 and he was able to be mine," Sexton said last July.
While saying that himself and Farrell had a "good relationship", Sexton also admitted they tried to "kill each other at the Aviva last time!"
Sexton went from hunter to hunted in Ireland's game with the Grand Slam-chasing England in Dublin last year. In the ninth minute, and after spotting Farrell make a nuisance of himself, Sexton bolted around the side of a ruck, made a beast of a tackle on the honorary Irishman and wasn't quick to let go of Farrell either as Ireland went on the attack.
Even though George Ford was at 10, it was Farrell at 12 who Sexton seemed to have the personal duel with.
In the second half, England responded by trying to do a hit-job on Sexton with some very questionable late tackles.
It wasn't hard to work out England's cues with Dylan Hartley's call-to-arms of "Sexton", which was clearly heard through the ref mic, giving the tactic away.
In that Ireland-England game TV producers decided to pick up where they left off when they used to show shots of O'Gara/Sexton on the bench after the other had kicked a penalty in the hope of catching a jealous rage being unleashed.
This time the close-up shots were reserved for the Farrells - when Owen kicked a penalty it was followed by a shot of Andy in the Ireland coach's box.
Today will be the second time Andy will be on the opposing Test team to his son, his first at Twickenham since his time at England.
"It's an odd situation for everyone but the two of them handle it very well," Ireland skills and kicking coach Richie Murphy said this week when asked what it's like to pick the brain of Andy about this son.
"They're close, very close, father and son. He treats him like any other player".
The Farrells have become used to managing their relationship when they're on the same side.
During last year's Lions Tour the players sent Father's Day videos to their dads. Owen didn't need to do that with Andy being defence coach but that didn't make it any easier for him.
"I didn't know when to say it (Happy Father's Day) to him because I didn't want to say it in front of anyone," Farrell said on the tour.
"I don't think I've spoken to him on this tour as my dad. There's lots of little conversations that go on in making sure that everything's covered."
Sexton will have one Farrell in his corner and the other Farrell in his sights at Twickenham today which all adds up to a unique situation.
Sexton probably knows Owen's game better than any out-half in the Six Nations but he would still want to drain every last detail about him.
For Andy's first Six Nations as part of the Ireland coaching team last year, it would be pretty understandable if Sexton initially felt a tinge of awkwardness in querying him about his son.
What we do know is that on previous evidence, such as last year's game at Lansdowne Road, Sexton and Farrell resumed their ferocious rivalry with no holding back.
Andy, obviously, has had an unparalleled influence on his son. Andy was also a hero of Sexton's growing up as he had an appreciation of Andy's toughness and uncompromising nature. Sound familiar?
When Owen and Sexton exchanged a few verbals at Twickenham in a Six Nations game in 2012, Sexton told him that just because his old man was a hard man, that didn't make him one. Touche. Sexton said he regretted it after.
It is the nature of the out-half position, especially in a game of today's magnitude, that the 10 can become the hero or the fall guy.
Ireland's 2009 Grand Slam game with Wales was ultimately defined by the actions of the two out-halves - the drop goal by O'Gara and the missed penalty kick by Stephen Jones.
Before getting carried away with celebrating at the full-time whistle, O'Gara consoled Jones and swapped jerseys with him.
Switched from 12 to 10 for today's game, Owen will look to replicate what Sexton did last year and stop Ireland from completing a Grand Slam on his home ground.
Sexton will look to replicate what O'Gara did and steer the team to a Grand Slam.
It was an O'Gara drop goal which bookended the '09 Slam. How sweet it would be if we can reflect on how it was Sexton's drop goal in Paris which kick-started this one.
Sexton knows how good he's going to have to be today because he knows how good his opposite number is.
As Andy said, the thing about Sexton is how he makes people around him feel.
That extends to all of us watching and hoping it will be a winning feeling today.