ASD includes a wide variety of characteristics. Some of these include behavioral characteristics which widely range from slow development of social and learning skills to difficulties creating connections with other people. Autistic individuals may experience these challenges with forming connections due to anxiety or depression, which they are more likely to experience, and as a result isolate themselves.

Other behavioral characteristics include abnormal responses to sensations (such as sights, sounds, touch, taste and smell) and problems keeping a consistent speech rhythm. The latter problem influences an individual's social skills, leading to potential problems in how they are understood by communication partners. Behavioral characteristics displayed by autistic people typically influence development, language, and social competence. Behavioral characteristics of autistic people can be observed as perceptual disturbances, disturbances of development rate, relating, speech and language, and motility.

The second core symptom of autism spectrum is a pattern of restricted and repetitive behaviors, activities, and interests. In order to be diagnosed with ASD under the DSM-5-TR, a person must have at least two of the following behaviors:

Repetitive behaviors – Repetitive behaviors such as rocking, hand flapping, finger flicking, head banging, or repeating phrases or sounds. These behaviors may occur constantly or only when the person gets stressed, anxious or upset. These behaviors are also known as stimming.
Resistance to change – A strict adherence to routines such as eating certain foods in a specific order, or taking the same path to school every day. The individual may become distressed if there is any change or disruption to their routine.
Restricted interests – An excessive interest in a particular activity, topic, or hobby, and devoting all their attention to it. For example, young children might completely focus on things that spin and ignore everything else. Older children might try to learn everything about a single topic, such as the weather or sports, and perseverate or talk about it constantly.
Sensory reactivity – An unusual reaction to certain sensory inputs such as having a negative reaction to specific sounds or textures, being fascinated by lights or movements or having an apparent indifference to pain or heat.
Autistic individuals can display many forms of repetitive or restricted behavior, which the Repetitive Behavior Scale-Revised (RBS-R) categorizes as follows.

Stereotyped behaviors: Repetitive movements, such as hand flapping, head rolling, or body rocking.
Compulsive behaviors: Time-consuming behaviors intended to reduce anxiety, that an individual feels compelled to perform repeatedly or according to rigid rules, such as placing objects in a specific order, checking things, or handwashing.
Sameness: Resistance to change; for example, insisting that the furniture not be moved or refusing to be interrupted.
Ritualistic behavior: Unvarying pattern of daily activities, such as an unchanging menu or a dressing ritual. This is closely associated with sameness and an independent validation has suggested combining the two factors.
Restricted interests: Interests or fixations that are abnormal in theme or intensity of focus, such as preoccupation with a single television program, toy, or game.
Self-injury: Behaviors such as eye-poking, skin-picking, hand-biting and head-banging.
Self-injury
Self-injurious behaviors (SIB) are relatively common in autistic people, and can include head-banging, self-cutting, self-biting, and hair-pulling. Some of these behaviors can result in serious injury or death. Following are theories about the cause of self-injurious behavior in children with developmental delay, including autistic individuals:

Frequency and/or continuation of self-injurious behavior can be influenced by environmental factors (e.g. reward in return for halting self-injurious behavior). However this theory is not applicable to younger children with autism. There is some evidence that frequency of self-injurious behavior can be reduced by removing or modifying environmental factors that reinforce this behavior. 
Higher rates of self-injury are also noted in socially isolated individuals with autism. Studies have shown that socialization skills are related factors to self injurious behavior for individuals with autism.
Self-injury could be a response to modulate pain perception when chronic pain or other health problems that cause pain are present.
An abnormal basal ganglia connectivity may predispose to self-injurious behavior.