I've been worldbuilding for a while and more so since it looked likely I'd get to DM for the first time. I provided a pre-session zero PDF to my players, containing 6 truths as well as both some details about common folk creatures and some blurbs about the more obscure races that the players decided to play as. (I've linked an edited version of that document here as a .PNG image.)
The world I have planned is a whimsical high fantasy, and I haven't said 'no' to anything so far. Players have thus proposed characters and we have a really bizarre roster as a result.
I'm planning to use 5E mechanics with custom races and mostly standard classes, spells, etc.
I'm going to run a session zero tomorrow and I just got a backstory from a player.
The character is a sentient animal who was made sentient by an elven sorcerer from another world/plane. That sorcerer forced them to watch and re-watch all of the real-world movies of a particular '80s action hero actor (Steven Seagal). The character, this sentient animal, now believes themselves to be this '80s action hero and that they did, in fact, experience the entire plot of each of these movies, including the ones where they died (but somehow survived).
If I wanted to meet-in-the-middle (whatever that ends up looking like), how would I approach the creative-differences topic?
I'd be singling them out, for sure. The other ideas for characters are nuts too. These things would seem whacky in wonderland; out of place in an OTT JRPG. But none of them insist on a multiverse. Much less one that involves the real world Hollywood action scene…
I'm tempted to just roll with it and see what happens, or see how the other players react to the ideas. Maybe it'll be fun and I'll stop caring about the details. If not (and presumably better to address it sooner rather than later), how do I change the player's mind without it sounding like "No. I don't like your idea."?
I've said no to characters before. Here's how it goes;
Once we get to this point generally either the player remakes a new character, or we work shop the specifics (the character gets renamed Riley and loses magic but still retains Sasuke's edgelord personality).
I definitely recommend talking to the player directly and being specific about the issues, I have found players generally understand the concerns or at least understand that you have the authority to say no. In your case I find it hard to believe that the player hasn't already anticipated that you will say no to their action hero critter.