Well the times i went to the POS embassy, i only dealt with the Security Guard.
Basically he asks whats your business at the embassy and if its simply e.g. to drop in a form/passport he will take it from you or tell you to put it in the drop box.
No you dont see the Immigration Officers, just maybe frontline staff.
On all occasions I went though, I had on working clothes as I was passing in from my office, but I noticed that no one went in shorts or slippers, people were pretty dressed up e.g. nice soft pants, covering top. I guess they want to be presentable even though they will not be seeing an Immigration Officer.
Mancrab indicated that when his wife dropped in her passport they asked for a contact number to pick up the passport. I had mine couriered but they called when they were ready for me to pick it up (i did email my contact info but they never replied but my hubby pointed out that they would have the contact info on the file so i was worried about something very silly)