system
Given the following post, respond as if you're commenting on Reddit:
human
CMV: a "good actor" has more to do with writing, directing and overall production than with a particular actor's skills.

Now I'm not saying that actors are not talented and that it is not difficult to act out difficult scenes. My qualm is with the separation of "good actors" and "average actors".

Matthew McConnahey is the obvious candidate who was thought of as nothing special until he had an opportunity to work with a good script, good production and good directing. Heath Ledger was another example. Robin Williams, too, would not have been thought as much more than a comedian until given the right opportunity. Or Will Smith. We may soon be saying the same for Jesse Eisenberg.

Given the quality of script, the careful directing, the number of takes and feedback and cues an actor may take to get a scene right, the music which can be an emotional trigger; it is hard to imagine any professional actor who could not be coaxed into giving stellar performances.

Obviously people will cite examples to the contrary, but to compare Gary Oldman who is widely regarded as a master actor, I've recently noticed the previously unremarkable aussie actor Joel Edgerton being cast as Rameses in the upcoming Exodus, and I think that it may not take much to make a competent actor "chameleonic".

Basically, I believe any competent or trained actor can be coaxed to give outstanding performances with the right directing and production. There is less variety in the capabilities of professional actors than the fandom of A-listers suggest.

Edit: to clarify my position: I think there is less validity to the claim "X is a brilliant actor and Y is a bad or average actor" than  there is to "X got a good role, script and director whilst Y has been unable to find a role or script he can really shine in."

Edit2: okay guys, took a bit of discussion but you did change my view. A few guys highlighted the inverse of my position, good movies dragged down by bad acting (Keanu in Dracula is one I'll never forgive) and average movies saved by an outstanding performance (no one would be mentioning Good Morning Vietnam without Williams). But most importantly was the talk of theater, which removes all of my talk of takes, editing and music and lets it be the bare actors. I never considered that, but it is absolutely contrary to my position and changed my mind. Dishing out the deltas for the 3 who changed my view.
gpt
I think there is definitely a major impact on an actor's performance when given a good script and good direction, but there is also a skill to the actor themselves and they way in which they go about their work.

A good actor will try to understand the character and their motivations, and will give feedback to the writers and director about how a line feels and sounds in that context. Also, a huge thing about great actors for television and films is their ability to self-choreograph otherwise improvised motions. Something as simple as how they hold a prop like a cigarette or a drink, and the timing of when they take a sip in the dialogue, and making that consistent goes a long way to making the editor's life a hell of a lot easier when editing for continuity. A lot of novice actors will move around at random, just reading the lines and trying to act as it comes to them. When you take the footage back to edit, it can often be a mess trying to line up shots where the actor's arms or legs are in the same position at the same word in a dialogue, and you end up having to edit around them instead of editing around how it would feel best. 

But of course, any production is a culmination of all its constituent parts.