Final answer:
Face-to-face communication is the most effective method for the Incident Commander and outside crews, due to the richness of nonverbal cues like body language and facial expressions.
Step-by-step explanation:
While radio communication is essential, face-to-face communication is the preferred (and most effective) method for communications with the Incident Commander (IC) and outside crews. Human communication relies heavily on language, which includes verbal, nonverbal, and written forms. Body language and facial expressions are among the nonverbal cues that make face-to-face interactions more informative and nuanced. This direct form of communication allows for a clearer and more immediate exchange of information, which is critical in emergency and operational situations where the ISO operates.