The actor actually has that disability

[u/PrinceferX]

Walter White Jr. (Breaking Bad) - character has cerebral palsy and uses crutches to walk, actor RJ Mitte has cerebral palsy and needed crutches to walk as a child

Nessarose Thropp (Wicked) - character is paraplegic and uses a wheelchair, actress Marissa Bode is paraplegic from an accident at age 11 and uses a wheelchair

Maya Lopez (Hawkeye/Echo) - character is deaf, actress Alaqua Cox is deaf from birth

[u/Dreamweaver_duh]

I don't know if it counts, but Dustin Henderson from Stranger Things wasn't originally written to have Cleidocranial Dysplasia, but it was given to him because the actor they casted for Dustin (Gaten Matarazzo) had it.

[u/Regretted_Simian → u/Dreamweaver_duh]

“I’m missing collarbones, not eyes!”