Snape's curious reaction in The Goblet of Fire

Question

Every time I reread The Goblet Of Fire I stumble on one sentence:

"I saw Voldemort come back!" Harry shouted. He tried to get out of bed again, but Mrs. Weasley forced him back. "I saw the Death Eaters! I can give you their names! Lucius Malfoy..."

Snape made a sudden movement, but as Harry looked at him, Snape's eyes flew back to Fudge.

It looks like one of those small details in a book that should be clear to the readers if they know the background. Something similar to the swift mention here:

For a fleeting instant, Harry thought he saw a gleam of something like triumph in Dumbledore's eyes. But next second Harry was sure he had imagined it

This detail about Dumbledore's reaction rises questions at first, but gets the explanation later in the story.

But in Snape's case it is not obvious at all (to me at least) what Rowling meant by this curious reaction. Did Snape react to Harry naming Death Eaters openly? Did he react to Malfoy's name specifically?

I feel like Rowling meant something specific here but I just can't get the point of it.

Answer 1

Snape and Lucius were old friends.

As I learned from this excellent answer on another SE site, there was more to the relationship between Severus Snape and Lucius Malfoy than just both of them having been Death Eaters and the former favouritising the latter's son for years.

• From Snape's very first day at Hogwarts:

  And Severus Snape moved off to the other side of the Hall, away from Lily, to where the Slytherins were cheering him, to where Lucius Malfoy, a prefect badge gleaming upon his chest, patted Snape on the back as he sat down beside him ...

  _{-- HP and the Deathly Hallows, "The Prince's Tale"}

• Narcissa also refers to them as old friends:

  "Severus ... please ... You are, you have always been, Draco's favorite teacher ... You are Lucius's old friend ... I beg you ... You are the Dark Lord's favorite, his most trusted advisor .. Will you speak to him ... persuade him."

  _{-- HP and the Half-Blood Prince, "Spinner's End"}

• And Sirius, admittedly in full taunt mode, calls Snape Lucius's "lapdog":

  "Tell me, how is Lucius Malfoy these days? I expect he's delighted that his lapdog's working at Hogwarts, isn't he?"

  _{-- HP and the Order of the Phoenix, "Occlumency"}

None of these quotes is really conclusive on its own, but taken together they point towards Snape having been a friend, perhaps even something of a protege, to Lucius during their Hogwarts years. To an unpopular boy from a poor background like Snape, any encouragement from a rich prefect like Lucius could have meant a lot.

Moving forwards to the time of HP and the Goblet of Fire, how is the relationship between the two now? Snape defected away from the Death Eaters at the end of the First Wizarding War, but presumably Lucius doesn't know that, and there's no reason why Snape would automatically hate all his old Death Eater friends just because he betrayed their leader who killed the woman he loved. It's quite possible that Snape and Lucius had continued to have a cordial friendship through all the intervening years.

Then, why the sudden movement? Surely Snape must have known that Lucius would still be a committed Death Eater when Voldemort returned, and therefore the two would be on opposite sides in the upcoming conflict. But it seems to be an involuntary reaction, so rationalisations like this don't necessarily work, even for such an accomplished Occlumens (implying immense self-control) as Snape. His thoughts might have been "oh no, my old friend's going to be an enemy! ... meh, I guess I should have known that". Or he might have hoped that Lucius wouldn't want to risk his cushy job and power at the Ministry for the chance of a different kind of power under a returning Voldemort.

Answer 2

Its unclear but it's possible (though unlikely) that at that point in the series Snape didn't know Lucius was a Death Eater.

"You must understand," said Karkaroff hurriedly, "that He-Who-Must-Not-Be-Named operated in greatest secrecy.... He preferred that we - I mean to say, is supporters - and I regret now, very deeply, that i ever counted myself among them -" "Get on with it," sneered Moody. "-we never knew the names of every one of our fellows
_{^{- Harry Potter and the Goblet of Fire, Chapter 30 (The Pensieve)}}

(Highlight is mine) this quote shows that up to this point it is possible (seeing as I cant find any definitive proof that they knew each others status) that Snape didn't know Lucius was a Death Eater and was merely surprised at Harry's statement seeing as he reacts directly after Harry says Lucius Malfoy. Another possible explanation could be that Snape and Lucius had a personal agenda seeing as they were close or admired each other.

"You are on probation!" shrieked Professor Umbridge, and Snape looked back at her, his eyebrows slightly raised. "You are being deliberately unhelpful! I expected better, Lucius Malfoy always speaks most highly of you! Now get out of my office!"
_{^{- Harry Potter and the Order of the Phoenix, Chapter 32 (Out of the Fire)}}

If Snape and Lucius had a close relationship its possible that they were working on something together and Snape was surprised to hear Lucius's name openly accused and moved to stop Harry but stopped himself because he was in front of Fudge.