Bob Ross would receive letters every week from his regular viewers. When a regular would stop writing to him, he would call them and make sure they're ok.
Bob Ross also once got a letter from a colorblind viewer who said they can't paint due to their colorblindness. He then did an episode where he made a painting entirely out of gray colors to prove that anyone can paint.
Mir. Rogers hand-wrote letters back to everyone who wrote to him.