If you want a hint as to how Ireland's game with Scotland will go today then watch Finn Russell's demeanour while 'Flower of Scotland' is being played.
Arise Clive Woodward for discovering his inner Poirot and finding this clue about the Scotland out-half for us. When he spotted Russell smile during Scotland's anthem before their Six Nations opener with Wales last month, Woodward came to the fool-proof conclusion that Scotland would get a spanking.
"In all my time coaching international rugby, I have never seen a player of mine laughing and smiling his way through the national anthems which Finn Russell seemed to do. That game was done and dusted before kick-off," mystic Woodward wrote following Scotland's thumping loss.
The stinker here for Russell is that his laughing and smiling conveniently fitted in with the fact that his head wasn't in the game against Wales. But when Russell gave Woodward's former team a good old kicking at Murrayfield two weeks ago, he seemed to throw that terrible sin of smiling back the direction of cranky Clive.
"Yeah, it's been a tough couple of weeks. But I'm going to keep smiling no matter what," Russell promised.
Meanwhile, on the flip-side, Johnny Sexton has been warned about the hazards of, er, not smiling.
"My concern with Johnny relates to his on-field demeanour and by that I mean the lack of enjoyment he appears to get from the game," Tony Ward wrote in the Irish Independent last October, a man who knows what it takes to be a top-class out-half.
"Sometimes I wish Johnny Sexton would loosen up a little. On a personal level, I like him a lot but smiling on occasion is not a sign of weakness. On the contrary, it will endear him to his team-mates and, dare I suggest, to match officials."
There's more than just a degree of a smile separating Sexton and Russell. If the impression of Russell is of a wannabe Lamborghini-driving, high-rolling risk-taker, it's a little different to the broad impression of Sexton.
Russell will move to Sexton's former club, Racing 92, this summer but what the pair really share is they've both delivered signature moments in this Six Nations: Sexton with his drop goal in Paris with a prelude that included that cross-field kick to Keith Earls, Russell with his pass into space and by extension Huw Jones at Murrayfield.
Today will be the first time since Murrayfield in 2015 (when Russell scored his first international try against Ireland) that Sexton and Russell will line up opposite each other in a Six Nations game.
Indeed, it will be Russell's first Test game at Lansdowne Road.
Sexton has played 71 times for Ireland, more than double the 35 caps Russell has for Scotland.
Sexton's reputation is formed but so is the less-experienced Russell's, such is the nature of out-half which is more defined by 'personality' than another other position.
It's an easy game to box off players into certain categories to enable us to get a grasp on what kind of player they are.
And so we'll get methodical Sexton v mercurial Russell, steady-hand-on-the-tiller Sexton v showman Russell, grumpy Sexton v smiling Russell and, of course, flaky Russell v fantastic Russell with one description in particular the go-to word for the Scotland out-half.
"The one word I hate to describe any fly-half is a 'maverick'. I hate it," former England player Ugo Monye said last month on the BBC Radio 5 live podcast.
"Maverick for me describes a player who can win you a game and lose you a game all within around three phases. I look at Finn Russell and he's a wonderfully talented player with a wonderful skill-set. But he's a maverick fly-half."
But players have personalities which, like everyone else, have fluid moments which go against their 'reputations'. Sexton shows he can take a risk as much as Russell but his overall game doesn't need high-risk moves for him to rule a match. Unlike Russell.
While he showed his mental strength with his performance against England after being castigated for his earlier displays, will he double-down on his high-risk play or realise a consistent 10 needs more in their locker?
On the flip-side, how will coming up against a player with a reputation for being a 'maverick' influence Sexton?
At the Six Nations launch in January, Joe Schmidt was doing interviews beside Gregor Townsend and Schmidt joked after that he was trying to listen to what Townsend was saying while talking himself.
Sexton may have the same approach with Russell today. When Sexton was ruled out of Ireland's game in Murrayfield last year, Russell said: "I never really buy into these personal battles. I don't look at it as a one-on-one test".
Sexton would probably say something similar in public but the impression of Sexton is that everything is personal and there is no damn way he will allow being out-played on his patch by Russell.
But the greatest rivalry Sexton has is the one with himself. His place-kicking was off against Wales but his open play was magnificent.
His pass to Jacob Stockdale which led to Ireland's opening try seemed cut out of an anger to make up for the earlier missed penalty.
In fact, he played like that throughout the game as if it's not just perfection he's chasing but self-preservation. Same with Paris when he missed the penalty and was driven to atonement with the drop goal.
Ireland and Scotland are at their best when Sexton and Russell are at their best which generally comes with the territory of being a 10.
But more than the other out-halves in this tournament, Sexton and Russell are an extension of their head coaches on the pitch.
Russell has been described as everything Townsend was when he played for Scotland. I always think the relationship between Sexton and Schmidt was best summed up by Brian O'Driscoll years ago when he joked that he would wonder if the JS initials at the end of a Leinster team-sheet were those of Joe Schmidt or Johnny Sexton.
Sexton v Russell will be box office today. But forget the comparisons and the sweeping categorisations. The most important match-up for Sexton and Russell is the battle they face with themselves.