ND Bill & Ted Headcanons
If you're asking me (and since you're on my site, you are), Bill and Ted are both totally autistic, and so are Billie and Thea. The girls getting their diagnoses when they're in elementary school is what finally help Bill and Ted realize their own - that they aren't just 'bad at being normal', they're totally neuroatypical!
Ted struggled A Lot with the transition into school when he was little; teachers let his mom come to class with him to keep him from having screaming meltdowns, but he wound up being held back and put into a Special Education class once she left. When Bill moved to San Dimas he was put into the same class for more subtle delays - like not being able to figure out how to hold a pencil or tie his shoes. They latched onto each other, became totally inseparable, and were able to move back into standard classes the subsequent year
Ted is also dyslexic; in the movies, Bill's always the one reading out of books while Ted paces or goes to ask other people for help, so this is a routine they've worked out to make sure Ted can also get information out of textbooks even though he struggles to read it for himself
They both actually have a solid grasp on music theory, but they kind of suck at learning guitar at first because of dyspraxia. Once they're able to get a better grasp at how to move / adapt playing to fit them better, they pick up newer instruments like nobody's business.
Ted wears a lot of layers because he likes the weight of them (he's also not great with temperature regulation, so being able to adjust layers helps a lot) and Bill has a lot of modded clothes - like his cropped shirt - because of texture hypersensitivity and poor impulse control. If he doesn't like the way something feels, he either chops that part off or cuts the article into small pieces so that no one can make him wear it.
Ted likes wearing his hair in his face because it's Stimmy and also because it cuts down on visual input and makes it harder for people to notice when he's not making eye contact. He also totally can't stand the sensation of hair clippers buzzing against his skull - after years of screaming meltdowns over it, Captain Logan finally gave up buzzing Ted's hair regularly, and he was finally able to grow it out.
Both of them really struggle to diversify their eating habits. They tend to stick to the same few Safe Foods and no one's ever paying enough attention to try and encourage them to broaden their horizons. Eventually they catch on to the benefits of multivitamins and it stops being such a huge deal.
Ted is Echolalia Central - picks up all of Bill's new vocab nigh instantaneously and is always repeating back things people to say to him to help him process what they're saying.
Even before they get their diagnoses, Billie and Thea grow up with WAY healthier sensory diets and coping mechanisms. Even before Bill and Ted learn there's a word for what they are, they were Intimately familiar with how bogus it was to constantly be forced into clothes/activities you can't stand, so whenever Billie and Thea displayed sensory aversions, the dads were able to help them avoid the trigger OR teach them how to cope with it better (like wearing headphones or long sleeves or something).
Billie took Forever to start talking - didn't really start using words until she was like four or five. The others were kind of worried, but she got along alright with Thea and noises/gestures, so they didn't feel particularly pressured to start Making her speak. By the time teachers started to push the idea of speech therapy, Billie was already on her way to total verbal skills, so that was that.
Thea likes baggy clothes because she can't stand clingy fabrics. Billie likes tighter clothes because she can't stand the way loose ones brush and move against her skin
Plus, check out the totally excellent ways Billie and Ted greet each other!! All the excitement and affection of seeing each other with None of the sensory discomfort of a full body hug!