So you think the game designers should have to write, "They can't even talk" to account for Other Activity on your Turn and include Surprise in their step by step combat to follow the intent of the rule?
No, I think what they wrote is just fine. It's the intent of the rule we disagree on, not that these other rules prevent its intent from being followed. The way I interpret it the rules all interact just fine.
A person casually conversing is "taking an turn" as they walk?
Is this person in combat? If so, then yes. Walking and talking are explicitly called out as things you normally only do on your turn, the exception being any reaction you take that involves talking. Normally, I would expect this would only be the verbal component of a spell cast as a reaction.
If not in combat then I don't know what sort of turn you mean.
You're basically saying that any time an NPC is casually talking, or for that matter walking down the street or moving in any way, they aren't surprised? You really going that far? Basically saying no one can be surprised unless they're standing still and are completely silent? Is that your interpretation?
Surprise prohibits movement, so a person who is surprised would not be able to walk down the street while surprised. It does not prohibit speech, since speech is not tied to either movement or action the way an object interaction is, so a person can still talk on their turn even when surprised.
You don't at all take into account that not being able to see your attacker before he acts would preclude that individual from acting? That the way the rule is written is an alternate way of saying, "they don't get to take a turn"? You're pulling stuff like they get to communicate even though they don't know the target is there from Other Activity clearly not in anyway intended to be used in the situation you are applying it to.
As a general rule I would expect it to apply to any situation where communication is not expressly forbidden. Since the ability to communicate is in a separate category from movement and action it is not ruled out when you are surprised.
Explain to me how they can be surprised because they don't know the target is there because the target has not yet attacked, yet somehow communicate as though they are aware? How can you have it both ways?
I'm not sure why you think communicating is dependent on being aware of the presence of an attacker.
How can a character act if he can't take actions and doesn't know the enemy is there?
Communication doesn't count as taking an action.
You pulling RAW out of the books means nothing to me, especially when you're pulling out rules like Other Activity out of nowhere because this game doesn't have free actions. It is very much implied that you don't get to talk in a manner that makes it seem as though you are aware of the attack on your turn.
Where is it implied that being surprised stops you from communicating on your turn?
I want to know how you justify it in the fiction. 5E seems very intent on focusing on playing in a fashion that does not break the fiction. You seem stuck on RAW interpretations of rules read in a very narrow, anal manner regardless of the fiction behind the rule.
To the contrary my reading is expansive, drawing on the ruleset as a whole to make my conclusions, whereas your interpretation of Assassinate relies on a narrow reading of the rule based on your own preconceived notions of how it should work without regard to how things as basic as combat are clearly meant to work.