iserith
Magic Wordsmith
The player always determines what the character thinks, does, and says, EXCEPT when those things are determined by the rules or the DM.
Short of magical compulsion which is an exception to the rule, the player always determines what the character thinks, does, and says. The DM can only describe the environment and narrate the result of the adventurers' actions.
They are always bound by the results of checks (or do players in your game simply disbelieve damage and hits away?).
That's not the same thing as the DM telling a player, for example, that he or she must have the character act as if the NPC is lying (or telling the truth).
A DM can choose to disregard the outcome of the rules/roles, and a player can choose to roleplay a scenario without invoking roles in many cases, but ultimately a player roles dice to try and succeed at a task or suffer the outcomes of failure. Why else are the dice there?
The dice are there to resolve uncertainty as to the outcome of a task proposed by the player when there's a meaningful consequence of failure. The DM determines whether or not there is uncertainty based on what the player has described as wanting to do. The player does not choose to make ability checks. That is solely the DM's call. If anything, it's not very smart play for a player to want to roll the dice as the d20 is very unpredictable. If success is the player's goal, then trying to avoid the dice is a better strategy.
The skill actually says “Your Wisdom (Insight) check decides whether you can determine the true intentions of a creature.” Note that it says the check decides what your character determines (which is telling them what they think). What exactly would your character’s thoughts be based on in such a scenario if not what they observed in their environment?
An ability check is not a task.
The ability check, to which the Insight skill proficiency may apply, resolves uncertainty as to the task the player proposed. The task is described if you continue reading the entry for Insight - "...gleaning clues from body language, speech habits, and changes in mannerisms." It has everything to do with observing another creature. The DM is telling you what you observe, just like when he or she is describing the environment, not what your character thinks.
A DM can’t FORCE a player to make a roll. A player can choose to make a role in pursuit of a mechanical task, which is why the rules exist in the first place. You can choose for your character that they believe someone is lying or telling the truth just as anyone can eschew engaging with reality when conjuring their beliefs and opinions. If you want to pretend your character knows what is what that is fine, but once you engage with reality is has a nasty way of conflicting with such beliefs.
A "roll" is not a task and players can't choose to make ability checks. That is the DM's call and he or she makes that call when the task proposed by the player for the character has an uncertain outcome and a meaningful consequence for failure.
At least, that's what the rules say. You can choose to play it some other way, of course.
To clarify my position, a DM certainly CAN choose what actions succeed or fail, but that involves disregarding the rules. That is something they are free to do (and should do in many cases), but it is not the default of each action generally. To make success and failure an arbitrary extension of the DM would create a lot of problems, not the least of which would be invalidating every mechanical choice that players made for their characters. To disregard a check here or there or ignore a rule when it suits a scene is fine. To reduce every character to an avatar of the player with no mechanical strengths or weaknesses would be playing a game other than 5e.
I'm sorry, but none of that is true, except perhaps at your table (and probably others). The rules come into play when the DM says they do since they serve the DM and not the other way around. The "default" is that the players say what they want to do and the DM narrates the results. Sometimes, when certain criteria are met in the eyes of the DM, the DM calls for a roll. A character's "mechanical strengths and weaknesses" come into play if and when the player proposes a task that has an uncertain outcome and a meaningful consequence for failure, which they will do quite frequently if the player is portraying a bold adventurer confronting deadly perils. But otherwise, success, failure, and uncertainty are all completely in the control of the DM.
Out of curiosity, did you play much D&D 3.Xe or D&D 4e?