Ovinomancer
No flips for you!
I'm a bit confused here as you're switching between GMs and players without calling out which applies, especially in the third paragraph -- are you talking about GMs here or players?I suspect that part of this difference in perspective is explainable by differenences in when the authorship is taking place. Making IC choices on-the-fly for a partially developed NPC is a simpler task than doing justice to a fully-fledged character in a pre-written work.
The improvising GM has much less material constraining their choices--the range of plausible IC choices is quite large. Additionally, the GM doesn't (yet) need to worry about future consistency and can even make IC NPC decisions where some of the rationale remains as-yet undetermined. For example, a GM can determine that an NPC decides to lie in response to a question about a crime not because the NPC is guilty of that crime, but because the NPC is trying to hide involvement in a different, as-yet-undetermined, crime.
By contrast, in non-improv acting the actor is constrained by needing to maintain fidelity to what has been written about the character, including material presented on stage thus far and the remaining script. (And any other works involving the same character.) On top of those constraints, the level of detail required is larger, particularly if the character is a main character (in contrast an NPC, almost by definition, will never be a main character). Plus there is more pressure too, as the stakes are much higher in theater than in a TTRPG.
So while both tasks involve an element of understanding human interaction, I think the comparison is complicated by the tasks being fundamentally different in scope, and the GM's simpler task being made still easier by also possing authorial power for the NPC.
Either way, I'm not sure I agree with the general premise here -- that improv acting is easier than scripted acting. They are different, yes, but I would strongly hesitate to say easier. You say that the scripted GM/player has more on their plate because they have to pay attention to the established character, what's already happened, and what happens next, but I disagree that this is so -- it's really only the character and what happens next that matters, because of the idea of the 'script'. The next thing that happens will happen regardless of past things, so it's only performance there that matters.
And all of this only goes to expression of character (I think @Bill Zebub calls this performance) -- you're just expressing a character for others and not exploring that character in any way. The alternative is exploring the character, which involves learning about the character as you play -- you're as much the audience as everyone else. The other aspect Bill Z brought up is how much you're imagining yourself in the position of the character, or how much your sharing the emotional freight of the character in the moment. This is orthogonal to expression/exploration, I think, as I can see doing this with high/low and with low/high respectively.