Behavior is defined as everything an organism does. Thoughts and feelings are behavior, too, but they should not be treated as special types of behavior.
The following is an excerpt from the web discussing this further: Skinner's radical behaviorism (1945) offered a unique conceptual framework for explaining human behavior that had no close brethren in Psychology. Skinner used the term radical to note the stark contrast between methodological behaviorism (i.e., the behaviorisms of Watson, Hull, and Tolman) and his approach. That is, this approach retained overt behavior as an important dependent variable of psychology while acknowledging the existence and significance of unobserved behavior (e.g., see chapters 15-17 of Skinner, 1953 Science and Human Behavior addressing self-control, thinking, and other private events). He did not, however, grant special causal status to such phenomena. That is, rather than place causal status in hypothetical entities or constructs, Skinner's radical behaviorism attempted to demonstrate orderly relations between behavior and environment . . . (https://www.psychologytoday.com/blog/radical-behaviorist/201002/the-radical-in-radical-behaviorism).
a) The concept may be closer in a person's mind because the concept is something the person thinks about a lot, and is therefore "chronically accessible;" you could even say that the concept has become part of a person's schema. Therefore, the person is more likely to perceive and be aware of this concept, even in ambiguous situations. For example, if someone is constantly thinking about rejection, then the person will be more likely to see and perceive it, even when it is not really there.
b) The person was recently exposed to something related to this concept or idea, and therefore is (at least temporarily) more likely to see it in the environment. For example, right after a person watches a murder mystery movie, a person may be in a slightly more suspicious frame of mind for a while when encountering new people; the person may temporarily be more likely to interpret ambiguous actions as "shady behaviors." Generally, though, these perceptions will fade after the effect of the movie has worn off and will not become chronically accessible constructs or part of a person's schema.
Lots of people fear snakes, spiders, public speaking, and heights. People also have a natural revulsion to things such as rotting meat and feces, which can be full of germs and harmful bacteria. Fears of each of these things conferred a survival advantage to our ancestors. Public speaking may seem different, as it cannot literally kill you, but in the past, being socially ostracized by the small band of 30 or so human beings with whom one lived could have meant death. It would have been difficult if not impossible to get along completely alone in the wild. Also, there were fewer people groups around, and joining another small band of human beings may not have been a possibility. Therefore, even the fear of social rejection has roots in survival. Also, being socially sensitive to others helps parents care for babies and small children; without a certain degree of social sensitivity to others' needs, we cannot care for children, and the species cannot survive. Therefore, social sensitivity, including fear of rejection, can be argued to confer a survival advantage just as avoiding poisonous snakes and spiders does.