Would you retain the comma in this sentence: the president called the protesters, terrorists? The statement has some obvious heat underneath it, and we often justify a comma we include by saying that it is emphasizing the word it isolates. But if we let emotion overtop the bounds of grammar (in prose, at least, for poetry is a different matter), we run an unnecessary risk of confusion: were there both protesters and terrorists in the crowd, and was the president calling them? Remove the comma and all is clear: the president called the protesters terrorists.
Transitive verbs denote action that is aimed at something, and that something is called the direct object. Language looks on the mental movements of thinking or considering or regarding every bit an action as walking or talking, but verbs like these provoke a peculiar grammatical construction called a predicate objective, which is to say little more than that they necessitate two direct objects, not one. If the board of trustees appointed Ruth president, the mental action of appointing (that is, making by agreement, or regarding) is pointed at Ruth, the direct object, and explained or particularized by the unimpeded addition of president, the predicate objective—predicate because it occurs in the predicate of the clause and objective because it amplifies the meaning of the direct object already present in the predicate.
Now commas can cause a host of problems. Many of us are happy to remember and apply with abandon a rule we might have learned in our foregone school days: place a comma where you breathe. Well intentioned were our teachers, no doubt, but that little directive has directed too many writers right over the cliff. Our emotions can affect the way we breathe, and if the pause that occurs in taking a breath to emphasize a word of sharp feeling—a word like terrorists—is translated into a comma, then we have separated the direct object, protesters, from its logical complement, terrorists, the very word that is needed to complete the thought. We do not want to think devoid of emotion, but emotion unbounded by structure too easily reads as overheated and therefore dubious.
It may be helpful to understand that the term comma derives from a classical Greek verb meaning to cut, so from its very conception, this mark of punctuation was meant to dissociate, not associate, words in a sentence. Verbs of naming or calling or choosing inherently associate one thing to another, and so by definition preclude the use of a comma. And the predicate objective need not only be a noun; an adjective may complete the meaning as well: the inspectors found the porch dangerous. Without the predicate objective dangerous being attached accurately, absent a comma, to the direct object porch, the meaning of the verb found changes entirely, from determined to came upon. Such logical distinctions are rooted in the punctuation.
The caution, then, is to keep your eye on structure, remembering that sentences must stand on solid grammatical ground before they can tower lofty, even excited, thoughts.