I've always sort of known this but just recently a particular direction in my acoustic guitar playing has really driven it home. The key to developing strength, dexterity and speed in your guitar playing (or indeed any instrument) is through developing finger independence. When you have finger independence your fingers just work better, period. And the best way to develop finger independence is through moving one or more fingers whilst keeping the other fingers fixed in position on the fretboard. For instance, if you're doing a left hand exercise which involves fingers 1, 3 and 4  on one string, keep your 2nd finger fixed in position on another string. If you're practicing a trill with fingers 1 and 2 then keep 3 and 4 fixed on another string. It really is amazing how much this improves your dexterity, especially with hammers and pulls. 
 
I've come to realize this over the last few weeks because I've been incorporating a lot of scale runs into my fingerstyle chord playing. That is,  I will play fast runs or trills whilst holding a chord down. For instance, try playing an ascending open Am scale starting from the open A string while keeping an Am chord held down as consistently as possible. When you're playing the scale notes on the A and D strings with fingers 2 and 4, fingers 1 and 3 remain in their positions on the B and G strings respectively. When you reach the G string, fingers 1 and 2 remain in their positions on the B and D strings respectively and so on.  
 
If you find this hard and you get into situations where you mean to move one finger but a different one moves instead, you don't have adequate finger independence. If you practice stuff like this all over the fretboard with different chord shapes (including barres), your dexterity will improve dramatically. Incorporating this kind of stuff into my playing of late has turbocharged my fingers! It feels like my hammers and pulls are as fluid and coherent when playing on heavy acoustic strings as they are when playing on my Telecaster. Try it! 

Hi,
 
In Brazil, a friend of ours that was into music, used to tap his fingers all the time, during conversation ... with one unusual detail ... they were not mechanical, as in "order" or numerically organized, so that at any time the third finger will skip, or the next time the 2nd finger does and I could never find a reason, or order for it.  Still, is, for my imagination a very weird thing, since most people tapping fingers or keeping up with a song, are usually very "metric" (numerical?), and I always thought what he did was not "orderly".
 
Is that weird, or just something like you suggest? I've always called it the "Stravinsky fingering"

Well unless he was keeping one or more fingers down while tapping the other fingers then it's not exactly what I was talking about, but having said that, I've always tapped my fingers in a haphazard fashion like that. I usually alternate my thumb with random fingers in succession, rotating my wrist back and forth while I do it. I can get some pretty good "drum rolls" doing it. 

One of the earliest exercise I learned from a friend of mine who could afford guitar lesson was supposed to be designed to help with that.
 
You'd fret 4 ascending semi tones on the 6th string, say 1-2-3-4. Keeping fingers 2-3-4 on the 6th string, you'd bring finger 1 on the 5th string and then the rest of your fingers one at a time, and so on. 
 
Or we'd start on the high E and work in reverse. That being said, I've never been too good at it - maybe I should have put more time into it.
 
Of course, I'm always tapping and flexing fingers, but I don't think it really helps much. I do know that I have better independence in my fretting hand because I can do this:
 

 
If I try with my picking hand, the rest of the fingers follow my pinky.
 
My fretting hands fingers don't crack half as much as my picking hand either (though this one also got better since I've started messing with chicken picking.
 
 

One reason my playing has never advanced over the past 20 yrs. is I just don't exercise. period. My 3rd & 4th fingers are tied together and refuse to work independently of each other even tho I can tap roll them 1 to 5 & 5 to 1. Someday I'll get over my laziness and fix it but 1st I have to get over my lethargic procrastination. Probably do that tomorrow.

I'm lazy too - and I hate to practice scales and stuff. But unless I'm reading a book or spending time with my wife, I pretty much always have a guitar in my hands. The best time to work on that stuff for me is while watching movies. Or Star Trek. Star Trek and scales have been associated in my mind since I was 18 or so...