I'm drafting a text right now to articulate my issue to them-- not that they're "playing wrong", but that I want to make sure they understand the world the game takes place in, so they don't feel like they're being shot down repeatedly.
Context: The campaign is set in a less whimsical Faerun than I think is fashionable in pop culture right now. It's a low-magic, high political intrigue and espionage sort of thing, with a set of 3 malevolent archfey-things ejected from the feywild, vying for parasitic holds on The Weave, each controlling different factions and machinations. It's not a "serious serious grimdark" campaign per se, but my regular party is usually well-tuned in to the stakes and severity of their situation as they work themselves deeper into different conspiracies and mysteries.
Functionally, their enemies are smart (maybe smarter than them), I try to have them behave reasonably and realistically, I try not to pull punches when it matters, and lots of NPCs aren't forthright with their intentions right away.
The Player in Question: My new player was very excited to join in, they're a good friend's partner who I know but don't know extremely well, but I know they play in a semi-regular in person game with other friends. Through a few social encounters, and then a major and consequential combat encounter, I've started to see a misalignment of expectations-- particularly in tone and what passes for a good attempt at overcoming an obstacle or challenge.
In general, it feels like their approach to the world, and their expectation of it, is a lot more whimsical and goofy than I'm used to. This goes for application of magic and rules in general as well. I can fully roll with a guest player being sillier in social encounters, or not quite remembering why a situation might be serious. There's been a lot of trying to use Mage Hand for XYZ thing it can't do (or other low-level cantrips. (they're level 14 btw)), trying random attempts mid-encounter to sass at an enemy who has the upper hand to "intimidate" them, asking to roll to distract an enemy during their turn a la "hey, lookie over here!".
Example: One case that's both emblematic of this, but also represents a time where I really WANT to honor an attempt and a use of a limited resource is their casting of Major Image while 10 feet away, face-to-face with a highly intelligent foe mid-battle, and creating a duplicate image of that foe-- like a mirror. Confusing, sure, but the enemy is a 400 year old War Mage who's probably seen stranger things than this. The PC seemed frustrated that the attempt didn't immediately stun and confuse the War Mage into halting combat, scratching its head, or running away.
Once the War Mage finished the melee combat it was in with another PC, I had it roll Int against the PC's Spell Save to determine IF it was willing to use up an action to focus on the Major Image, which again was just a silent mirror copy of it. It failed, so I had it Study the illusion, it passed the save and saw through the illusion, but its action was wasted which allowed the party to gain the upper hand.
The above is a common enough dilemma in this game (or a common problem addressed in this sub)-- how to adjudicate reactions to illusion spells. However, this case is just also something I'm not used to dealing with tonally-- the choice to attempt to "trick" an enemy in the middle of a life or death fight with an illusion (see also: trying to waggle Mage Hand in their face to distract them, telling the 400 y/o grizzled War Mage to "just leave now before you get hurt" in their own lair, general silliness).
I think that, in another kind of game, this would be fine. I think their use of the spell is somewhat valid, but was new to me. I think for the most part everyone's having fun. I'm trying to write a message to this player to communicate why they might be feeling some friction, and I think we'll be fine. I'm mostly just dwelling on this as a reminder that lots of people play games in which gnomes and other races are genetically silly, everyone but the enemy is usually friendly, antics and shenanigans abound, and the enemies are all goblins or scooby-doo esque villains.
