Short answer: Just do it

Long answer: For the nurse/doctor scenario, put the uniform on, walk up to where he is (as in the waiting room) call his name and invite him into the exam room. Begin by taking his pulse, looking inside his mouth (say: ahhh), and, pretending to look in his ears. You can then ask him to remove his shirt(s) so you can listen to his heart, check the sensitivity of his nipples, check his skin condition by brushing your hand over his upper body. Next, things become a bit more dicey. You have to ask him to strip down to his under ware and recline on the exam table (bed). As you continue to rub your hand over his lower abdomen in order to check his internal organs, slip your fingers inside the waistband of his under ware and go exploring. Ask him to slide his under ware down in order to check his scrotum, testicles, and penis however seems appropriate. If he has an erection--comment favorably about it and tell him you would like to check its functioning. If he somehow remains flaccid, inform him that you have to check his penis and which will require that you cause it to become erect (etc.) Take the exam as far as you want. If you know how to prolong his excitement, anticipation, and, agony, then pause periodically. If you want to make him climax, do so. If you want to become "involved" with your patient, then perhaps this will require you to have intercourse while mostly dressed and above him in the Woman Superior/Cowgirl position. The rest I leave to your vivid imagination.

When you have completed your exam, ask him if he has any questions or concerns, then instruct him to get dressed and to see the office staff to arrange payment. You should then get up and leave the "exam room".

My 'roleplay' scenarios are somewhat different - mainly because there is no costuming involved. I do more 'de-costuming' in fact.

To begin I come in very close as if going to kiss him on the neck below his ear and whisper "Do you trust me?". Upon receiving his affirmative, I then sneak a hand to his shirt front and begin undoing his buttons while running my lips along his jawline from around his ear to his lips and undo his belt buckle while kissing him and all with excrutiating exquisite slowness - and then I go on from there.

The basic idea here is to always be doing two things at once while staying physically close to him - well inside his comfort zone - so his senses are ignited and his desires overwhelm his ability to reason or object and he's distracted by my proximity and perfume.

Dancing Doc described my nurse fantasy almost to the letter. Another couple of things you can do is check his balls by feeling all around them. When he's erect, say you need to measure his cock with a ruler. Comment on how hard and big it is. Say it looks delicious and ask if he would mind if you took a taste test of his cock. I also like a prostate exam, so get him on his knees and lube up his ass hole with your finger, and then dive in and massage his prostate. He should drip some cum if you massage his prostate right.

For a secretary role play, wear a thong underneath your short skirt, and leave the top few buttons of your blouse open. So when you bend over in front of him, he can look straight down your cleavage and see your boobs. You can also turn away from him and bend over straight legged to file some papers in the bottom drawer, letting your skirt ride up so he can see your thong. You can also sit on his lap to take "dick-tation". Also, try leaning into him with his face in your cleavage as you reach for something behind him.

When he's good and hard and you're ready for sex, go back to filing papers in the bottom drawer and leave your legs spread and wiggle your butt. That should be invitation enough for him to do you doggy style.