To avoid author intrusion, you have to be careful, too, not to show anything your viewpont character can't see. For instance if the viewpoint character describes their own "blushing red cheeks" she can't see that unless she's looking in a mirror.
On the other hand, you can describe gestures and inner feelings and emotion in a viewpoint character. For instance, the protagonist can "hope her excitement didn't show on her face," or "feel the heat of embarrassment burning on her face."
When we show a detail only other characters can see, it's called "author intrusion" because the writer is telling the reader something that the VIEWpoint character can't possibly SEE. One way around this problem (I still catch myself doing it, so I surely know how to fix it) is to use one of the character's other senses, to get the point across. Your protagonist can't see her cheeks blush without a mirror (and that's done too often, and too often badly), but she might "feel her face grow hot" or her "try to swallow back a tide of embarrassment and wish she could drop right through the floor." This was the hardest viewpoint lesson of all for me.