The following is adapted from Reclaiming the Principalship by Tom Marshall.
A fine line is drawn when the principal, who is the primary evaluator of classroom practice, acts as the coach. Coaches work with teachers when teachers are most vulnerable: teaching often less-than-polished lessons; working toward new competencies, but not yet mastering them. If we want teachers to take risks and grow, we must learn to notice things with a different lens—a coaching lens.
This is tricky work. You never really can separate yourself, leaving the evaluative part outside when you enter the classroom. Just the same, you never really abandon the coaching part of you when you enter a classroom, either. Part of instructional leadership is knowing when to let each part of you take the lead.