Can a POV character make assumptions about what another character is feeling? For example, could you have character Jim think that character Sally is surprised? It's Jim's POV, so he doesn't know for certain that Sally is surprised. Who knows, maybe she has a gas pain. But, is it really unrealistic for his POV chapter to have a line like: Suprise crossed Sally's face. (Or something like that.) To me, this is clearer to what Jim thinks instead of just describing Sally's features. I ask, because I've seen criticism of doing that.
Sure. Here's an example: Surprise flashed across Sally's face. Oh? Jim wondered, did you finally figure it out? He narrowed his eyes at her and waited for a response. It came in the form of a rumbling, gurgling eruption that exploded out of her mouth in a titanic burp. His mouth twitched as they both laughed. "Guess you got Dad's gas issues," Jim said as Sally flushed with embarrassment. "Oh laugh it off, Jimmy. Your guts doesn't feel like someone's driving a rusty knife into them and twisting it." Not sure if this works, but that's what I envisioned.
I would argue that this is not only preferred, but necessary to keep POV consistent. You're exactly right--there's no way Jim can know what Sally is thinking or feeling. He can only guess. And whether or not he's right could make all the difference in terms of character development or plot. That's why close POV is so useful--it allows you to color the narrative with Jim's biases, whether they're true or whether they're a warped view of the world. I've always been an advocate of this style. When writing it, however, I would be careful not to make it read like a POV break. You do want to write without being wishy-washy--as in, not everything has to "seem" or "feel like"--but you also don't want to confuse the reader into thinking we're in Sally's head at any point. There are many ways of doing this. You can outright say that surprise crossed her face but then have Jim consider it with the next few thoughts, like how @Link the Writer showed above. Surprise isn't always a complicated expression--sometimes it really is easy to see that someone's surprised. It just comes down to the writing. Keep in mind, however, that sometimes the clarity isn't preferred--maybe Jim should be guessing; maybe the uncertainty is important. And this can also slip into telling/summary in a situation when showing is more useful. In the end, it's all a balancing act.
The only thing that I would suggest is that whenever you include a character's unique perspective you also include the other character's reaction. It can be confusing if you don't make it clear what the other character is actually thinking or feeling. This could also be crucial to specific scenarios.
Characters make assumptions about other characters all the time - sometimes those assumptions are right, and often they are wrong. The wrong ones can be a lot of fun.
I think it's extremely important to show the character's POV. The character is narrating, thus the character will see their world through their own eyes, thus so must we, even if Sally isn't surprised.
Of course he can, if his thoughts about what her reaction means is more important than the reaction itself. Although I Would prefer to see the sign of Sallys surprise through Jims eyes without any interpretation, so I can draw the conclusion myself, but that is just me. If it's important that he's assuming she's surprised you should stick with the way you said.
