Was Fenster's famous, incomprehensible line ('Hand me the keys, you cocksucker') truly an offhand remark, or had he already seen through something and was issuing a warning in a way that others couldn't understand?

Ha, this is practically an evergreen topic among fans of The Usual Suspects. Whenever I discuss this film with friends, Fenster's "quirky" line almost always comes up.

Regarding your question, we can examine it from two perspectives: the "real-world level" of film production, and the "interpretive level" of the movie's narrative.

Real-World Level: This Was Actually an Improvised "Stroke of Genius" by the Actor

From a filmmaking standpoint, Fenster's mumbled line can almost certainly be attributed to actor Benicio Del Toro's own improvisation—a soulful addition he brought to the character, rather than a premeditated warning written by the screenwriter.

Let me explain:

Character Development: After being cast as Fenster, Del Toro felt the character was somewhat "functional" in the script—merely one of the five required members. To make the role more distinctive and memorable, he devised this incomprehensible, mumbling speech pattern. He reasoned that a guy named "Fenster" (meaning "window" in German) wouldn't sound like a native English speaker, so he gave the character an exaggerated, unique accent.
On-Set "Accident": The most fascinating part is that during the lineup scene at the police station, when Fenster delivered his iconic line—"Hand me the keys, you fucking cocksucker"—in his distinctive slur, the other actors (like Gabriel Byrne and Kevin Pollak) couldn't help but break character and laugh. Director Bryan Singer was reportedly annoyed at the time, but later realized this genuine, spontaneous reaction among the actors created a fascinating tension—a group of wanted criminals daring to act so defiantly and mockingly in front of police. Thus, many of the shots we see in the film, where the others struggle to suppress laughter or even chuckle, were authentic reactions captured in the moment.

So, from a "real-world" perspective, the creation of this line was more like a classic cinematic moment born from an actor's brilliant improvisation and an on-set accident. Its primary purpose was to establish Fenster's eccentric, rebellious, and authority-defying personality.

Interpretive Level: Why Do People See It as a "Warning"?

However, the charm of your question lies in how it invites a second interpretation—the "Warning Theory."

Though its origin might have been simple, within the film's narrative, it can indeed be interpreted with deeper meaning. Why?

The Film's Suspenseful Atmosphere: The Usual Suspects unfolds like a giant puzzle, compelling viewers to scrutinize every detail for clues. Fenster was the first among the five to want out and the first to be killed. This makes him appear more attuned to the looming danger than the others.
The Symbolism of "Keys": In a mystery story, the word "keys" is loaded with meaning. It easily evokes thoughts of "the key to solving the puzzle." So when Fenster slurs "Hand me the keys," viewers naturally wonder: Did he see through something? Was he using coded language understood only by the group, hinting, "We're being played, find the 'key' (way) out of here"?
The Omnipresence of Keyser Söze: The film revolves around the enigmatic Keyser Söze, a ghostly figure looming over everyone. Under this intense pressure, any unusual line can be interpreted by viewers as a signal related to Söze. Fenster's line perfectly fits this "chilling upon closer thought" vibe.

Conclusion: So, What Is It Really?

Personally, I lean towards believing this is a beautiful misunderstanding and a case of "unintended brilliance."

The Truth: An actor improvised to make his character more interesting.
The Result: This improvisation, being so uniquely executed and appearing in a puzzle-filled film, was imbued by audiences with meaning far beyond its original intent.

This is precisely the greatness of The Usual Suspects: While the creators might not have intended such depth, the atmosphere they crafted compels viewers to interpret and speculate, infusing every detail with possibility.

So, you could say: Fenster (or rather, the actor) was simply being cool and provocative in that moment, but his unintentional act, through the audience's lens, became an incomprehensible warning foreshadowing a vast conspiracy. This also makes his death feel even more tragic and fated.