In Inception when they embark onto the plane, Cobb covertly gives a sleeping drug to Fischer in a glass of water. In the next scene we can see that the air-hostess is also working with them. So why does he have to give the drug secretly when the air-hostess could have done this?
The flight attendant is clearly under their instruction and operates the dreaming equipment. In the Inception universe where Extraction is clearly an illegal profession, it is unlikely that she would be unaware that what they are doing is likely to be illegal. So I do not entirely buy the argument that asking her to drug Fisher would be a crime whereas the other functions she performs would be not considered a crime. However I would agree that perhaps Cobb is minimizing her duties, keeping within the core team whatever can be performed.
My other speculation on this point is that Cobb is on a flight to the USA, where he is wanted for murder, and he is also wanted by Kobol Enterprises. He is perhaps suspicious, and wants to keep control of the drugs to himself and not give them to someone outside his core team. He has struck up a conversation with Fischer and is going to 'toast' Fischer's father's memory to encourage him to take the drug early in the flight. A simple mistake by the attendant passing Cobb the drugged glass or a deliberate double cross from someone who might drug him for a reward would ruin the attempt at Inception - leaving him in the USA where he would be arrested. He might want to drug Fisher personally to make sure it is done right.
The flight attendant isn't exactly on their side; they own her airline. Instructing her in what to do while they are performing their tasks is one thing, and, given that they are flying as executives, not really likely to be that unusual. But asking her to commit a crime, drugging a person without that person's knowledge, is an entirely different issue.
Caveat: I haven't watched the movie for a few months, so I may be forgetting details.
Not only is Cobb paranoid, he's also not very trusting; especially of people he doesn't know. He is constantly getting distracted by Mal; even when Ariadne confronts him about this and tells him to walk away, tell the others, give up some of his responsibility etc., he doesn't. He wants to be in total control of everything. It stands to reason that he'd want to be in charge of everything he possibly can. If he could turn on the machine in his sleep, he probably would have done away with the air hostess completely.
While the answers that Cobb is trying to just make sure and wants everything perfectly under his control are probably correct, there might still be other reasons to directly approach Fischer, as mentioned in my answer to a similar question on Movies & TV Stack Exchange:
Before Cobb hands him his drink, he brings the conversation to Fischer's father, saying what just seems a bit of flowery speech:
Cobb: You know, I couldn't help but notice, but you wouldn't happen to be related to the Maurice Fischer, would you?
Fischer: Yes, he... he was my father.
Cobb: He was a very inspiring figure, I'm sorry for your loss... Here you go.
Fischer: Thank you.
Cobb (raising his glass): Hey, to your father, may he rest in peace.
(Fischer nips and stares onto his drink)
This might as well have been a little preconditioning. The movie plays to a large degree with common ideas we think to know about dreams, the time dilation, the reluctance to notice divergence from reality, the lack of knowledge when it began... In the same way you sometimes tend to dream about exactly the stuff that just happened to you before falling asleep (which is why you should rather learn a language before going to bed instead of watching horror movies ;-)).
So bringing the conversation (and thus Fischer's thoughts) to Maurice Fischer and the imposing footprints Robert has to step in might serve to prepare Fischer's mind for dreaming about his father and his legacy and might thus help to make him a little more susceptible to the actual Inception.