The F.A.R. is 91.519 PASSENGER BRIEFING (and is applicable to all 121 and 135 carriers). It states:
(a) Before each takeoff, the pilot in command of an airplane carrying passengers shall ensure that all passengers have been orally briefed on -
(1) Smoking: Each passenger shall be briefed on when, where, and under what conditions smoking is prohibited. The briefing shall iinclude a statement, as appropriate, that the Federal Aviatioin Regulations requrie passenger compliance with lighted passenger informatioin signs and no smoking placards, prohibit smoking in lavatories, and require compliance with crewmember instructions with regard to these items;
(2) Use of safety belts: Each passenger shall be briefed on when, where, and under what conditions it is necessary to have his or her safety belt and, if installed, his or her shoulder harness fastened about him or her. This briefing shall include a statement, as appropriate, that Federal Aviation Regulations require passenger compliance with the lighted passenger sign and/or crewmember instructions with regard to these items;
(3) Location and means for opening the passenger entry door and emergency exits;
(4) Locations of survival equipment;
(5) Ditching procedures and the use of flotation equipment required under FAR 91.509 for flight over water; and
(6) The normal and emergency use of oxygen equipment installed on the airplane.
(b) The oral briefing required by paragraph (a) of this section shall be given by the pilot in command or a member of the crew, but need not be given when the pilot in command determines that the passengers are familiar with the contents of the briefing. It may be supplemented by printed cards for the use of each passenger containing -
(1) A diagram of, and methods of operating emergency exits; and
(2) Other instructions necessary for use of emergency equipment.
(c) Each card used under paragraph (b) must be carried in convenient locations on the airplane for the use of each passenger and must contain information that is pertinent only to the type and model airplane on which it is used.
So no, not before the aircraft moves, but before the aircraft takes off. As for the flight attendants being more "vigilant", all the lead can do is tell the captain "we're having some trouble with the video, but it's in now". If he elects to take off before it's conclusion...it's his responsibility, not the flight attendants. And pilots are so embarassed to get on frequency and say "uh, even though we're number one, we're not quite ready, can you give us about five minutes out here to finish up our safety demos?" They should, be they won't...especially the egos of an international MD11 crew....
Don't blame this on the flight attendants.