Most managers I have worked with were promoted because they were good in front of doctors.
So on a double visit, the temptation is obvious. Show the rep how it is done.
Sometimes that is exactly right. A rep who has never seen a good opening cannot build one from a debrief. But I have also seen a manager take over a call and leave the doctor with one clear message: the rep is not the person to talk to.
How I use modelling now:
Watch first. Let the rep lead at least one call before showing anything. You cannot model a fix for a gap you have not seen.
Agree the moment in the car park. Which call I will lead, and why. Never halfway through a call.
Model one part only. The opening, or one question. Not a whole performance.
Hand it back. The rep leads the next call and tries the same part. We debrief their attempt, not mine.
The one thing I avoid: stepping in because a call is going badly. That rescues the call and costs the rep their standing with that doctor.
“Show one part, then hand it back. The doctor's relationship stays with the rep.”

Lesson 11 of 23 in Step 4: Coaching, with the full guide and a checklist for this week.