My observations after watching 2 years of college ball:
The trigger is pulled pretty quickly and brutally on non-performing players. A bench guy may get a shot, but he won't get it for long, so it's "seize the moment". Fielding errors are especially not favored, and seem to be a quick ticket to the pine if they happen with any frequency. A place is always found for a hot hitter, so if a kid gets a chance, and hits, he'll get more. Once a spot is lost, the chance to regain that spot isn't easy. The chance to get back in the lineup is somewhat dependent upon how the replacement does. I've seen kids start the season hitting well, and play almost all the innings, and then when the hitting tailed off, the innings tailed off as well.
It seems brutal at times, but college players learn pretty quickly that the margin for error is pretty thin.
Good analysis on pitchers above-look at the innings and you'll see a core group of pitchers gets the bulk of the work, and success is rewarded, just like hitters. Front line starting pitchers seem to naturally get more chances to prove themselves, and to perhaps work out of a slump. Lesser used pitchers may get their shot, but there won't be a lot of tolerance for poor performance. Success is rewarded, and good performance will earn more playing time.