Playing the cello...the Left Hand...
I have discussed bowing the cello with the right hand, now I will comment on the left hand. That poor appendage has a VERY DIFFICULT JOB!!!
There you are, sitting there, with your legs spread apart and a big hunk of lumber between your thighs, and you look down at about a yard of dark wood that has no frets or ANYTHING on it to mark anything, and the note you need to play is SOMEWHERE on it. So, your left hand has to know where the notes are...and it has to get RIGHT ON THE 'fingerboard' in the RIGHT PLACE!!! If you are even a fraction of an inch off the right place, the note is out of tune. Plus, the distance between the notes change as you move your hand down the neck of the cello to play higher pitched notes. The distance between whole note steps can be about an inch and a half in first position, so there is space between the fingers. But get into the higher positions, and the distance gets less and less, to where your fingers are right beside each other playing whole note steps further down the fingerboard, and you have to pivot your fingers and just lay down the tips in the higher positions. And, again, there are no markers so finding a note in the higher positions is quite a task. There was a famous asshole Conductor who, when he was auditioning violinists, would tell the person to play an E natural in 7th position, and if the person didn't just drop his finger in the right place on the first try...well...Adios...
So, there you are with your hand on the neck. The thumb is behind the neck, and the fingers are sitting above the strings, and you drop your fingers to hit the notes. So, that is why string players sound so bad at first, their fingers have to find a note and learn the distance between the notes, and that takes a LOT of practice for the muscle memory to kick in. PLUS, sometimes a note is marked to be sharp or flat, so you have to drop the finger above or below where the 'Natural' note is. Or, because of the Key Signature, you just have to know which notes are sharp or flat because they aren't marked.
Then, you have to condition your fingers and wrist...they get tired at first when you take up the cello, and can cramp. Also, you MUST develop thick calluses on the tips of your fingers, soft mushy fingers can't hold the note steady. You also, when you get advanced, develop a large callus on the outside edge of your thumb, because you lay your thumb across the strings in the upper positions to hold the hand steady, and to act as a 'nut' if you have to play a lot of notes. My calluses were so thick that I could pick up VERY HOT objects with my fingertips and not get burned or feel any pain. Also, it was HELL getting my daughters hamster off my left first finger when it bit me...it's long teeth got caught in my callus. I actually had to just tear the callus to pull the little rodent off of me!
Then, there is...VIBRATO!!!
That is where the note gets very rich because the finger is rocking on the note, slightly varying the pitch. It looks like the cellist is wiggling her fingers, but that is NOT what is happening. Actually, the finger in planted on the note, and the HAND is moving up and down, parallel to the fingerboard. That takes a LOT of practice...to learn vibrato. Also, there are various sounds you can produce with vibrato. Holding the finger down and moving the hand slowly produces a very deep and dark sound, lightening the pressure and moving the hand quickly makes a more frivolous sound. And, you have to hold your hand and wrist steady, if the wrist gets loose the sounds get very interesting. As a beginning student, I actually strapped a ruler to the back of my hand and my forearm, so my hand wouldn't start something on it's own. That did work, the hand and wrist learned to stay together when playing the cello.
Then there is the pressure on the fingers. I was always inclined to stamp my fingers down hard, to settle the note. That works when playing slow, but you have to just lightly touch the string when playing fast. Like when you sprint, you don't stamp your feet, that slows you down.
The BIGGEST challenge is deciding how to 'finger' a passage...that is...which finger do you use to play a note? You have four of them, and ANY finger can play a note. In difficult passages, I would actually write a number above a note to remind me which finger to use. Usually, I would just write the number '1' to show that I needed to play the note with my first finger, but often I would write the numbers 1 through 4 to mark a complex passage so each finger knew what to do. I picked that up from the Private Teacher I had when I re-started the cello when I hadn't played for years due to my work requirements. He was an Englishman who had a terminal case of OCD, and he would always mark EACH NOTE with the finger he wanted, and whether he wanted to do an up-bow or a down-bow. When we were stand partners in an orchestra or a musical, I would spend a few hours carefully erasing ALL of the fingerings and bowings he marked in the part before returning the part to the Librarian. Once, I didn't do that, and I got a HUGE scolding from the Librarian AND the Conductor, and went to the office during a lunch hour with a truly humongous eraser and fixed the problem.
So, there you are, with a cello between your legs, and each hand is doing a totally DIFFERENT THING, and they have to do it TOGETHER!!!
And 'Lefty' has to find the notes on the blank fingerboard...