In Inception, why does Cobb slip the sedative into Robert's glass of water himself, taking considerable amount of risk at getting caught? Why didn't the Air hostess get the pre-drugged water for Fischer? Is there any reason for this?
Cobb wanted to be absolutely sure that the drug was in. There was just too much at stake to leave something like that up to the hostess.
I asked the same question when watching it lately and while Origin's answer may be right in that he simply wanted to get absolutely sure, there might still be other reasons involved for directly approaching Fischer. 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.