Actors should have no problem being in front of a green screen. I have seen more plays with minimalist sets than with elaborately-constructed sets; the fact that Elsinore was nothing more than three columns didn't screw up Hamlet, nor did reducing Rome to a single ladder screw up Caesar's eulogy. As John Rhys-Davies said in the RotK commentary, (paraphrase) We are actors, we are paid to live and work in the imagination, this is what we do (end paraphrase). I would expect any A-grade actor, supported by his or her director, to do green screen work nearly equal to his or her on-set or on-location work.
That caveat, of course, is key. As an actor is asked to imagine more outlandish premises, he will need more direction. I'm sure if Liam Neeson were told to act out a situation from a well-established play/movie or something from one's common experience, he could do it on autopilot in front of a green screen. He could be a taxi driver, or a passenger on an airplane, or an alien abductee. Everyone knows what you do in that situation, even if you've never driven a taxi or been on an airplane. But it's much harder when one is told, "You'll be playing the hologram of a Jedi master projected from Tatooine to Coruscant," or whatever. Do you play it as if you're in the room, talking to people face-to-face? Or do you adopt a more formal, telephonic demeanor? Are you comfortable, or trying to finish the conversation quickly? Is the transmission crystal-clear, or are there reception problems? Is there lag? Etc, &c.
And I'm sure acting against a green screen -- or a tennis ball, or what-have-you -- is much harder than even that.