LENS • It can be injuries, suspension or, in the case of Roy Hodgson on Thursday, desperation.
Circumstances can force managers to rip up plans, make changes, throw everything in the air and see how the pieces land.
Hodgson would not be the first England manager to start a tournament with one team and make up another on the move - it worked for Alf Ramsey in 1966 - though few have had to adapt in quite such strained circumstances.
England trailed Wales 0-1 at half-time, staring at a crisis.
Defeat would not have eliminated Hodgson's side from Euro 2016 or brought an immediate and ignominious end to a long career. But his reputation was at stake and England had to win to silence Gareth Bale and Co., who had queried their passion and superiority.
Asked if he had rolled the dice when he threw on two strikers at half-time, Hodgson claimed: "No, it was a controlled decision.
"It seemed to me that we were on the ball so much over 90 minutes, so it's good when you've got a few players who can do something different around the penalty area and ask further questions of their defence."
But some could see the beads of sweat on his brow. Two substitutions at half-time, both strikers?
He had never made a half-time change in his three tournaments as England manager. Moreover, England had never made a double substitution at half-time in any major tournament.
It worked because Jamie Vardy and Daniel Sturridge scored the goals that spared England and Joe Hart's blushes, after the goalkeeper was beaten by Gareth Bale for the opening goal.
It worked because Marcus Rashford helped to drag opponents wide when he, too, was brought on as Hodgson made full, belated use of his attacking roster.
England went within a whisker of a damaging draw. It was a flawed performance but they did enough attacking to deserve a lucky break.
A change of pace, and Sturridge's poise, won this game. It should help England to conquer the group if they beat Slovakia on Monday.
Now Hodgson just has to work out the best team on the evidence of two matches that have shown a manager yet to grasp his strongest XI and a clear identity.
There is no doubt now that England's attack will be different against Slovakia.
Harry Kane, who appeared laboured, looks sure to be dropped, or rested.
Sturridge has to play. If Hodgson sticks to 4-3-3, the Liverpool forward can take one of the flank roles. His clubmate Adam Lallana pushed his case by saying: "He (Sturridge) doesn't need too many chances before he gets his name on the score sheet."
But there could be no place for Lallana himself, as he creates more smoke than fire, and Raheem Sterling, whose confidence is dented.
"It would be foolish in the warm glow of a win, and feeling happy with four points, for me to make too many statements about what my plans are for Slovakia," said Hodgson. "We want to win that game. We'll be working hard to try and do that."
THE TIMES, LONDON, AGENCE FRANCE-PRESSE