na-adjective
Describes a person or approach that takes action rather than just thinking or planning. Often used for someone who is hands-on or energetic in pursuing goals.
彼はとても行動的な人で、すぐに新しいプロジェクトを始める。
He is a very active person and starts new projects right away.
行動的なリーダーシップが求められている。
Proactive leadership is required.
na-adjective
behavioral
Technical or academic use, referring to behavior or conduct. Less common in everyday conversation; 行動的 is more often used in sense 1.
行動的な問題を抱える子どもへの支援が必要だ。
Support is needed for children with behavioral problems.
活動的 describes being energetic and busy with activities, often physical; 行動的 is more about the tendency to act rather than just being busy.
Compound of 行動 (action, behavior) + 的 (-like, -ive). The suffix 的 is productive for forming adjectival nouns from nouns.