Autism is a spectrum, everyone does NOT act the same. In severe cases, the person does not speak and they "stim" a lot, such as rocking back and forth and/or making noises. They also act younger than what they are (a person in their 30's may act like a teenager and throw tantrums). On the mild side of the spectrum, it's possible that they don't know how to socialize and make friends. They also may say or do things that are "weird" or "offensive" to people without even realizing it. If someone gets upset at them, they won't understand why. They also aren't able to pick up on social cues or understand jokes/sarcasm. A lot will obsess over a specific item, hobby, fictional character, etc. Again, not everyone acts like this, it always varies from person to person.