Filmmaking evolved from live acting, so it has inherited a lot of methodology and mythology from that source. Over the years, cinematography, screenwriting, production, directing, and especially stunts and effects, have evolved away from those theatrical origins. There's no question that these arts in filmmaking are a distinct entity, certainly related to the theater yet distinct enough that no one would question that they are their own disciplines.
It seems to me that acting has not separated as much. Certainly, if you speak to actors, or those who teach acting, they'd be inclined to disagree by pointing up the many differences of technique they study for theater versus film. But I think these differences are treated far more like refinements of a common base of technique, like the differences between watercolors and oil -- or maybe more like the differences between watercolors and charcoal sketching, or watercolors and digital art. But not like the differences between watercolors and sculpture, or between watercolors and violin. But I suspect most actors who are studying acting techniques study a core set of techniques, probably for a fair amount of their education, before they even start to get into the differences in much detail.
Specifically, there's an assumption that anyone who can act in one format can probably work in the other. Certainly some actors prefer one or the other. And maybe some people are better at one than the other. Theater requires more skill at improvisation, and requires acting to be more holistic -- you have to be emoting, hitting your mark, remembering your next line, listening for your cue, dealing with props, etc. simultaneously and in real time, while in film you have some opportunities to split these things up and think about only some of them at a time. Film requires more of certain kinds of emoting and voice quality due to the close-ups and two-shots, more dynamic movement in some genres, more fluidity of dealing with a story out of sequence, and lots of adaptations to effects and cinematography considerations. But anyone who's good at one should be serviceable at the very least at the other.
Actors often assume that theater is the more unforgiving and thus more challenging. One usually assumes that a well-respected actor of the stage can do the screen with only a bit of retraining, but when an established screen actor (particularly those that seem to be getting by mostly on their looks!) tries at the stage, we wait and wonder: will they do okay (and thus prove they always had the acting chops all along) or bomb (and thus prove they were just coasting on how much "easier" film is)?
I wonder, though, if there are actors that have some genuine, powerful, rare talent for the kinds of acting you do on a screen but who suck on the stage for reasons that have nothing to do with a lack of talent, who are getting short shrift merely because of the historical legacy of acting coming from the stage. Acting is such a delicate art; no one really knows why it works when it does. How hard is it to imagine someone with an awe-inspiring talent who happens to also be very shy about people, so has their talent drain away on stage because of the proximity of the audience, not because of the more demanding timing of stage acting? Or someone whose finely detailed nuances of expression show up nicely on the two-shot but are completely lost from the third row?
What's worse is, are there people with that kind of talent, people who might be the best film actors in the world, who didn't make it that far because their initial exposure to acting usually comes from theater, or from an education that assumes learning stage acting is part and parcel of acting. Is it possible today to grow up and become an educated, trained, skilled actor without ever having to go through stage acting? And even if you did, would being terrible at the stage make you looked down on so much that an awesome talent on the screen would be overlooked?
My guess is that actors and non-actors would have opposite reactions to this post, and will chide me for being ignorant in opposite ways, and both of them would tend to assume their reaction is the only logical one to have.