You play as doctor, your partner plays as patient.
You play as patient, your partner plays as doctor.
You play as nurse, your partner plays as doctor.
You play as nurse, your partner plays as patient.
I think you get the idea.
Pretty much you take on a role. Just as you take on the role for anything in life, you pretend to be another role until the activity is done. Role playing only lasts from minutes to hours, but is usually not done long-term, especially in the "playing doctor" scenario.
Most people play the doctor/patient routine, where one person plays patient by laying on a table, nude, and the other "doctor" examines then. The play is sexual so the "doctor" tends to examine the body parts that are usually hidden under clothing. If you are female and have gone to the gynecologist then your partner would probably play that role and examine parts here-and-there (of course, not inserting things that would cause harm to you).
Take for example: Doctor does a breast exam. Doctor makes you open your mouth so they can check the back of your throat. Doctor presses on your body here and there to make sure nothing hurts. Doctor listens to your heart beat. You tell doctor what may hurt, what they may want to examine, etc.
Dress is usually according to role play. A doctor would have a stethoscope and gloves, at least, if not full scrubs for an outfit. They can be removed if the activity becomes more sexual or moves on to something out of this role playing.
It's easy to become carried away when you're role playing so boundaries tend to be set far in advance. If things are not wanting to be inserted then it should be said clearly and understood before engaging in the activity. A key word is usually used just in case things get crazy and you need it to stop--after all sometimes "no" in the doctor's room doesn't mean "no" in reality for some. People tend to safe words they wouldn't regularly say, just in case.
Please note that if you are going to engage in sexual activity then precautions to prevent pregnancy and STD transmission should be used. STDs can travel to your skin even if a condom is used so please get yourself and your partner screened for STDs beforehand. Condoms should be nearby, even if you feel there is not going to be any penis-in-vagina intercourse happening--sometimes things can get carried away.
Remember that in the heat of the moment you can do things you were not prepared for. Think about things clearly before going through with them. Sometimes you'll say, "Yes!" right now when an hour later you wish you would have said, "NO, not right now!" Talk with your partner well in advance to make boundaries clear, wants and desires noted, and a sturdy foundation set before engaging in activities that could result in babies or illnesses.
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