Mephistopheles - How Should He Be Played?

The portrayal of Mephistopheles should be as a transition from the character of a servant to that of master through the duration of the play, but also as a transition from something of a father figure to an enemy.
In the beginning, as he is summoned, Mephistopheles seems pliant and willing to become whatever Faustus desires of him.  In Act One, Scene Three, he is summoned by Faustus's incantation, and in line 36 could be played as being friendly or jovial as a demon.  He will then seem to the audience like someone who will be a friend to Faustus and try and guide him onto the right path.  The audience will remain aware that Faustus is committing sins by raising demons, but will perhaps be glad at least that he seems to have got such a nice one.  Lines 41-43 can be played as regretful, but in a way that comes across as paternal towards Faustus.  The following line (46) can also add to the image of perhaps being able to protect Faustus from the worst of the devil's wrath, being that he can "come hither of [his] own accord."  This can suggest that he is not totally under the control of the devil, which will subsequently make his eventual betrayal so much worse, as the audience has felt they can trust him.
This should continue through all his actions until the end of Act Three, Scene Two.  The vast majority of the scene should be played as the paternal, friendly figure, but at the last moment after Faustus runs off beating the friars, Mephistopheles should stop just before he exits, turn to the audience with a spectacularly evil look on his face, and then exit.  This will give the audience a clue into the real personality of Mephistopheles, but only enough to confuse them about his real intentions, not let them know that he is going to betray Faustus.  This will then also add an element of darkness to an act that is otherwise totally comic.
In Act Four, Scene One, the beginnings of the evil in Mephistopheles can be let out a little more. When Faustus tells him to go fetch Alexander and Thais, he can walk away, staring back at Faustus with the same evil look.  It is possible to have Faustus perhaps rub the back of his neck as though it is hot and aching as Mephistopheles stares.  When he returns with the two demons, he will have returned to his normal self.  At the end of the scene (110) he should look angered by the fact that Faustus wishes to walk on foot, and perhaps throw a tiny curse in his direction when he is not looking that causes Faustus to double over, leaving a diabolical smile on Mephistopheles's face.  He will, however, then run up to Faustus to make sure he is alright, returning to his jovial facade.  The curse will somewhat have weakened Faustus, causing him to sleep a little in the next section.  The audience should be becoming wary of Mephistopheles at this point, as his actions get crueler.  For the rest of the act, however, he should continue playing the paternal figure.
In Act Four, Scene Two, Mephistopheles will again hit Faustus with another small curse that weakens him in much the same way as the last time.  He will leave the stage when commanded (15), glaring at Faustus's back, but it will only be when he returns to the stage that he will hit Faustus with a curse that sends him to his knees, doubled over and gasping for breath.  Mephistopheles would then run to him and help him up with a caring expression, supporting him as Faustus presents the duchess with the grapes.  This gives the audience a clear understanding of the double life that Mephistopheles is leading; the life of a servant (which he despises) and that of a demon, one which is gradually coming out more and more as he cannot contain himself.
In Act Five, Scene One, Mephistopheles should play all his lines with as much poorly disguised contempt as is possible. As Helen passes over the stage, he could send such a strong curse towards Faustus that Faustus collapses into a chair behind him, clutching his chest and gasping for breath again.  The scholars will not notice because they are too captivated by Helen to be paying any attention, and should assume that Faustus has fallen into his chair at the sight of a woman so beautiful.  As the old man enters subsequent to this, Mephistopheles should cower in a corner, to show the audience that God still has precedence over hell.  It should be a real effort to pull himself over to Faustus in his chair to give him the dagger, which should be presented with real menace and hatred.  When Faustus delivers lines 63-66, Mephistopheles should return his lines with disgust at his abandonment of Lucifer and show his true colours; he is the real master in this show and Faustus will know it now.  Mephistopheles should shout and scream his lines, causing Faustus to cower before him and do whatever he says.  Lines 89-90 should be spat at Faustus with total sarcasm.  Bringing Helen in through a door to stage right, he should exit through this door, slamming it behind him.
As Faustus is dragged into hell in the last few lines of Act Five, Scene Two, Mephistopheles should just look on from front centre stage with a blank facial expression as Faustus screams and begs for salvation.  After he is gone into hell, double doors at the back of the stage through which the devils entered should be left wide open.  Mephistopheles will walk to the doors, turn to the audience, smile as a scream from Faustus is heard, and shut the doors on hell.

In this instance, the perfect actor to play Mephistopheles would be this fella.

I honestly think that Stephen Fry would be the best man on the PLANET to play such a character.  He is wonderful at being jovial but also a really good actor, as can be seen in the episode of Derren Brown in which he appeared.
For anyone who is unaware of this episode, Derren apparently trained an ordinary but incredibly suggestible bloke to be an assassin, for reasons which I think had something to do with a conspiracy about some kind of murdered important person.  Anywho, to prove that you COULD hypnotise someone into doing this, he got this suggestible man to shoot Stephen Fry.  Which he did.  Little bit scary, but there you go.  Stephen Fry does an EXCELLENT impression of a dead person.
ANYWAY, Stephen Fry is cracking for this kind of character because he comes across as such a lovely, happy man, but I can imagine him getting really very frightening if he was crossed.
Hence Stephen Fry.

Of course, all of this is immaterial if Faustus is to be played like this guy.

In this instance, Mephistopheles should be expressionless and emotionless throughout.