When we are playing classical chess, it is rather common to play some moves without thinking too much. However, you must be careful, for in apparently equal positions you might find a tactic that you wouldn't have found had you played rather quickly.
One game of Carlsen-Keymer in the world cup caught my attention. After losing the previous game, Carlsen had to win the next in order to keep in the competition.
Carlsen made a blunder, and Keymer didn't play the right move which would have won him a pawn and granted him a basically unlosable position, which would be easy to play. Besides that, Keymer played the wrong move in less than a minute.
https://lichess.org/study/y1qnrT8x/gy6xxcS0#34
If keymer thought more time, he would have found knight takes e4 and it would be extremely difficult for him to lose that position. If that happened, Carlsen wouldn't have won the world cup, the last important title that was left for him to win!
So, it is fundamental to pay attention even in positions that seem peaceful. The game looked like it was going to be an endgame, which helps to explain why black didn't find qxc3, and focused more on trying to completely equalize the position.
It is fundamental to look for tactics and opportunities even when they seem fleeting, always keep this in your mind!
