One way to show that the action was taken without giving it much thought is to describe the consequences before the action itself:
Original:
I heard the distinctive sound of a flint wheel. Without thinking, I jumped. I felt a pipe hitting my ribs. As I lay on the ground I heard rumbling in my ears but there was no explosion.
vs
I heard the distinctive sound of a flint wheel. I felt a pipe hitting my ribs. As I lay on the ground I heard rumbling in my ears but there was no explosion. I had only jumped.
In action scenes this can help build the tension by leaving the reader momentarily guessing what exactly has happened.
But it can work just as well in a more mundane scenes to show the lack of deliberation on the narrator's part:
I was bitching about my mom not letting me go to the party and thoughtlessly mentioned: "But you're lucky though - no one can tell what to do anymore!". 723
<PERSON>'s face suddenly grew pale and she stared daggers at my skull. I heard <PERSON> saying: "Dude! Not cool!"
vs
I was bitching about my mom not letting me go to the party when <PERSON>'s face suddenly grew pale and she stared daggers at my skull. I heard <PERSON> saying: "<PERSON>! Not cool!" What had come out of my mouth was: "But you're lucky though - no one can tell what to do anymore!"
This should also work in a more distant third person narration (played here for humor):
<PERSON> stumbled out of his bed, grumbled a greeting to the rug (the cat was away), made himself a tea and mindlessly poured it straight into the bin. He stared at the light brown liquid swirling in the black bag for exactly 57 seconds before muttering "Just my kind of day!"
vs
<PERSON> stumbled out of his bed and grumbled a greeting to the rug (the cat was away). Then he stared at a light brown liquid swirling in a black bag for exactly 57 seconds. He had made himself a tea and poured it straight into the bin. "Just my kind of day!" he muttered. 406