Behaviorism has gotten a bad rap. In various histories of the discipline of psychology, it tends to get reduced to a blank slate, anti-mentalist, it doesn't exist-if-you-can't observe-it, stimulus-response-reinforcement-operant conditioning, useful-for-its-time-but-now-obsolete, cartoon of its former self.
For me the essence of behaviorism is none of the above. It's about the ABCs: Antecedent, Behavior, and Consequence. The idea is you can't understand why a person behaves in a certain way without having a firm grasp of antecedent conditions, the behavior itself, and what happens afterwards. We're talking about what triggers a behavior, what constitutes the behavior, and what happens after the behavior.
A definition of behavior: any action of an organism that changes its relationship to its environment. The locus of action and its environment can be internal or external. For instance, a cognitive act* - say, redirecting attention away from a troubling subject - may change one's mood. An external act may also change mood - say, intentionally sitting up straight and taking a deep breath.
A behavior can be a micro-behavior in a hierarchical chain of goal-directed behaviors. Micro-behaviors self-adjust in response to ongoing input from their environments. The ABCs of micro-behaviors often happen below the threshold of consciousness. A tilt of the head, mirrored (or not) by another.
Brains and agents are prediction machines. Behind every behavior is an anticipated consequence. A pay-off. Before every behavior is its trigger. The trigger hints at a pay-off and sets the whole ABC sequence in motion.
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For the purpose of this discussion, I'm defining "internal behavior" as cognitive and attentional moves that involve top-down control processes in the brain.