Autistic individuals take literal meaning from what is said. If you say, "Lilies are my favorite flower" and the autistic overhears you telling your grandma, "I love begonias, they're my favorite" the autistic individual may think you are lying or being secretive. They will not understand why you told them one thing and your grandma something else.
If you say, "I'll be home at 5:15" and you get home at 5:45, the autistic individual may be agitated. Not only because it altered their schedule, but because they may feel that you lied to them about when to expect you home.
This is because autistic individuals take statements literally and expect that you intend your statements to be literal at all times.
1. Don't make absolute statements. For example: "I never eat fish", "I always love a good movie", "I like to be alone when I get home from work". Autistic individuals may take these statements literally and act accordingly for the rest of their lives. Autistics have incredible memories, especially for perceived social rules. If you say, “I don’t like onion”, the autistic individual may never offer you anything with onions ever again. This can be applied to more serious situations as well and can create significant disconnects between the autistic and neurotypical individual.
Sometimes absolute statements just slip out. They cannot be fully avoided.
2. Take them back. If an autistic person seems to be responding to certain situations the same way every time even if their response doesn't work, they may be relying on a "rule" they learned from something you said (or something their mom said or a friend said, etc). Ask them why they do a particular thing and see what their response is. They may respond with something like, "because you said you always want flowers on your birthday" or a similar, absolute rule.
Do not argue with an autistic individual and tell them you never said the statement. Autistics have very keen memories of their perceived rules. Even if you don't think you said it, just accept that in one way or another (to the autistic person at least) you did.
Getting defensive will not help the autistic learn that when you said "always" you didn't really mean "always". Just accept that they picked it up somewhere, correct them, and move on. Once corrected, the autistic individual will probably never fall back on the perceived rule again.
Television and Movies
The media can be a huge source of perceived social rules and literal statements. An autistic man may watch a show about dating in which the hosts state, “Always give your girlfriend flowers on Valentine’s Day”. He may take that as an absolute statement and apply it to every Valentine’s Day with his girlfriend, who doesn’t understand why the only thing she ever gets are flowers.
If the autistic person seems to be picking up absolute social rules from outside sources, it may be best to offer them new rules. For example, you could say, “It’s important to give people things that they love for Valentine’s Day”.
Autistic individuals tend to absorb rules easily, so if there’s something about their behaviour that needs to be changed, offering a new rule can help achieve that. It’s important not to abuse this or attempt to “trick” the autistic individual. Only help them with social rules that are actually true.