But aren't those spacemarines formatted by years of training/brainwashing in order to get them being perfect "don't think but kill for the emperor" troops ?
I can see a captain/chaplain/high rank guys having some kind of personality so the other spacemarines may look up to him in awe ( "For the Emperor and for The Guy With A Personality !" ) , but i imagine if a non-ranked spacemarine display some level of individuality a bit too much or even worse may start to not follow the chapter master/captain/etc.. orders to the very letter, it could be easily percieved as some chaos taint/heretic thought or whatever else.
There's two sides to this.
One is the exterior, monolithic view of Space Marines as stamp-molded warriors of the Imperium, each in lock step with the other and the Codex Astartes. It's the view most Imperial Citizens have of Space Marine, and that non-Space Marine fiction espouses.
The internal view, i.e. Space Marine fiction, definitely lets people have distinct personalities. While they all may fight and die for the Emperor, one might be an arrogant jerk. One may be such an Emperor-botherer they take a hardline stance on everything, including the preservation of human life. One is always invariably the soft heart, a Space Marine with the heart and conscious of a normal human. (To a degree anyways.) This is usually the author trying to come at the moral/ethical quandary angle of living in the 40k universe. And there will usually be the hothead, the bloodthirsty assault guy, the grizzled Sergeant who will probably die at some point so the main character gets all angsty.....
So, as usual, all view points on a thing in 40k are right depending on your vantage point. Consider like the military in the real world. When you walk by 1000 guys in full battle dress presenting arms, you're INTENDED to see them as faceless cogs of the war machine. That's intentional even today. So when you see 100 Space Marines lined up, helmets on, bolters at the ready, it's pretty reasonable to see them the same. They WANT you to see them that way because it makes them scarier and more intimidating. But just like soldiers loosen up around each other, out of view civilians, Space Marines are the same way. They have a legend to live up to in public.
There's also been a trend in recent years to humanize Space Marines a lot more. Older Space Marine fiction leaned heavier on the "hypno-indoctrinated warrior monks who live for nothing but duty and battle." That tends to make for some pretty boring novels though (or super grotesque ones), so they've loosened the leash and allowed them to have personalities to the extent it makes the fiction readable.