Hi !
It makes me think that, to a smaller extent, we tend to (no counsciously) mimic external people quite easily. We tend to catch accents, body language, oral language specificities of people we spend time with really quick : an afternoon can be more than enough. It's even stronger when we're with people we really, really want to get along well with, or if we admire them a lot.
Not all of us do that, it seems to be a "social alter" thing. Erdian is the best, I'm the second one, Kal and Claude sometime pick things too. We tend to see it better with people really different from us, because we will say specific words or use specific gestures we never use, but those people do.
It happens also with TV shows if we developed a specific interest on this TV show, or on a particular character. The first few days/weeks after we found of the show, there's a lot of things we will "catch" from one or more characters, specially if the show has a lot of episodes... If fades with time, usually when the specific interest lose strength, but can come back if this interest is triggered enough.
Last specific interest in a TV show we had (if you don't count the "Ladybug" interest of our Littles, from which we didn't catch anything as they aren't social alters) was with David Tennant playing the 10th Doctor... we caught a lot of weird things, mostly non-verbal, and even if I love the 10th Doctor and Tennant, I'm glad it didn't last more than few days.
The worst was with Damon from The Vampire Diaries, when we were a teen, because it involved more than one of us (for very different reasons

) and we caught a lot of oral language specificities, to a point it was obvious to my family. Few of them stayed even years later. For example, now all my family casually answers "yep/yop/nope" because of us (back at the time, no one was using them in France).
The funnier was with a character our SO created. Erdian and I love this character a lot and want to know more (everything) about him. We have moments where we read again all that have been written about this character just to find clues about his past (we have a file with all the infos we gathered) and usually, after we did, Erdian will have a bit of the character's mannerisms.
I call it our "social chameleon" thing, and see it as a mix of autistic traits and the necessity to fit in quickly we had growing up, as my parents moved a lot. DID just make it more... interesting

You never know which alter will develop an interest into a character, or what they will catch from them.
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French person with ADHD
Functional multiplicity, former partial DID